A counter-terrorist police raid on the wrong flat results in the death of an innocent man. DS Arnott refuses to take part in a cover-up and is recruited to AC-12, an anti-corruption team led by Superintendent Hastings. Arnott's first investigation is focused on DCI Tony Gates, the senior officer of TO-20 within Central Police CID, who has a suspiciously high clearance rate thought to be due to laddering - attributing numerous different charges to the same defendant to increase an officer's number of successful cases. Initially, Hastings interviews Gates on a minor charge - failure to report accepting a free breakfast for apprehending a mugger outside a local café. Gates reassures his lover, Jackie Laverty, after she claims to have hit a dog after driving drunk on her way home from a work party. Laverty reports her car as stolen and Gates helps cover up the incident by faking a break-in at her home. When he hears of the cases that have come in overnight, he discovers that Laverty killed a man, not a dog as she claimed. Fleming, currently undercover in CID, informs Gates that crime statistics on the Bog estate are on the rise, before requesting to join TO-20. Gates asks his team to survey two suspected drug dealers who are operating from a derelict house, but Cottan and Kapoor leave their post early and the two dealers are found murdered next day, with their fingers cut off. Laverty makes a statement about the hit and run to an intelligence officer, but fails to disclose her role. When the victim is identified as Gurjit Patel, Laverty's accountant at her business Laverty Holdings, Gates gains access to the police records, and deletes the missing persons report - but not before Arnott manages to take a look.

Gates continues his investigation into the double murder of the two suspected drug dealers, and forces a less than co-operative witness to talk by threatening to label him a police snitch. Meanwhile, PC Bannerjee and PC Larkin respond to a silent alarm at a block of flats and catch a burglar, Lee Plater, in the act. During interrogation, he confesses to a number of other offences, including a break-in at the house of a pensioner whose complaints are being dealt with by Fleming. Arnott interviews Nadzia, the waitress who gave Gates the free breakfast, and she reveals that a tall, slim and dark-haired woman was in the cafe with Gates. A search on the police database reveals that a number of burglaries committed by the offender caught by Bannerjee and Larkin were at properties owned by Laverty Holdings PLC. After visiting a hairdressers, one of the properties owned by Laverty, Arnott suspects that Laverty is running a money-laundering operation. The investigating team realise that the dealers' murderers cannot have found what they were looking for, but find a secret hiding place, where they suspect the drugs may have been stashed, which has since been emptied. Gates confronts Laverty over the death of her accountant. She invents a story but Gates realises that she was laundering money. Gates arrests her, but on the way to the police station she persuades him to release her. Meanwhile, another drug dealer is found dead on the estate, hanging from a lamp post with his fingers cut off, but Fleming realises that Gates has disappeared and informs Arnott. While Gates and Laverty are together in her house, masked intruders force their way in. Gates is knocked unconscious, Laverty is murdered, and Gates's fingerprints are placed on the murder weapon.

Arnott arrives at Laverty's house, following a tip-off from Fleming, to find Gates apparently searching the property, claiming that he has come to arrest Laverty. Whilst at the property, Gates tampers with evidence, hiding his whisky glass and wiping his prints from the bottle just as his team arrives. Arnott demands that Forensics dust for prints in the hope of proving that Gates was at the property before Laverty disappeared. Hastings and Arnott haul Gates in for questioning. Gates confirms that Laverty was an old flame, but denies all knowledge of the money laundering scam. His hesitation raises their suspicions. After Hastings is instrumental in taking Gates off the Laverty case, DI Ian Buckells is appointed to be in charge of the case. Fleming looks at CCTV from the night of the murder of the two drug dealers and finds footage that suggests the killers circled the area in a car with cloned (false) number plates until Cottan and Kapoor left. Gates frantically tries to dispose of the whisky glass from Laverty's house. The cloned car is spotted, and Gates and Morton give chase. During the pursuit, Gates is kidnapped and taken to a warehouse where he is shown Laverty's body and told to stay quiet, or the knife with his fingerprints on will be handed to the police. Arnott acts on a tip-off to say the whisky glass is hidden in a storm drain, based on false information that Gates gave to three of his officers, including Fleming, to test their loyalty. Kapoor is implicated and forced to leave Gates' team. Arnott messages Hastings and Fleming to tell them he is not the right person for the job and Gates has won.

Fleming suggests to Arnott that the link between Laverty's money laundering scam and Gates may be his children's expensive school fees. The school confirms that Laverty paid the deposit and the first full term in cash. After suffering a crisis of conscience, Arnott agrees to stay with AC-12. Hastings interviews the rest of Gates's squad over the laddering charge. Morton refuses to comment, Fleming sticks to her cover and backs up Gates. During Hastings' interrogation of Cottan, about the night of the surveillance on Greek Lane, Cottan claims that Gates ordered him and Kapoor to go off duty early, creating a gap in surveillance that enabled the killers to strike. Forensic evidence at the crime scene and Arabic books belonging to the victims lead Gates to concoct a cover story regarding a possible terrorism plot. Gates finds the location where he was held, but Laverty's body has disappeared. He receives another call from 'Tommy', who informs him that he wants Arnott out of the way. Morton catches Fleming with two phones; realising she is an AC-12 mole, he assaults her and drives away. Arnott receives a phone call from one of Laverty's tenants claiming to have information linking Gates to the gang. When Arnott arrives at the meeting, he finds Gates colluding with the gang. Before he can escape, he is ambushed and tied to a chair, with his right hand in a vice. Gates drives away.

Gates returns to rescue Arnott from the hands of his torturers. Arnott, by now convinced that Gates is innocent of the murders and can lead him to the killers, pretends that Gates did not play any part in his kidnapping. With her cover blown, Fleming is forced to leave TO-20, bringing her into conflict with Morton again. Cottan is assigned to replace Gates on the triple murder case, while Arnott's former counter-terrorism Chief Inspector is brought in to oversee the operation into apprehending the suspected terrorists. Fleming interviews Ryan Pilkington, the teenager who acts as a courier for the organised crime group, who denies any knowledge of 'Tommy' or the murder of Laverty, but reveals to Fleming that it was Gates who rescued Arnott. Fleming, upset that her colleague lied to her, informs Hastings. Arnott uses Morton to set up a midnight rendezvous with Gates, who tries to convince Arnott that he has never 'crossed the line', and that he was forced to turn rogue by the sequence of events. Arnott persuades Hastings to let him use Gates to get to 'Tommy'. Hastings agrees to put a tap on the phone network used by 'Tommy' until one of his many phone numbers is re-activated. Arnott primes Gates, who convinces his wife to let him spend one final hour with his children. When the phone number is re-activated, Gates, Arnott and Fleming trail 'Tommy' to the Edge Park golf club. Gates arrests Tommy Hunter, who reveals that his boys were responsible for the murder of Laverty and the three drug dealers. Hunter claims that the murders were committed by 'an inside man, someone special to him' who 'always follows orders'. As Arnott and Fleming catch up with them, Gates, realising his career is over, tells Arnott to say that he was killed in the line of duty, before committing suicide by walking in front of a lorry. After Hunter is arrested, Cottan asks for a few moments with him, and privately tips him off about the terrorism theory.

In the epilogue, Gates's family receives a death-in-service benefit of £107,000 plus a pension for life. Arnott testifies, but his former counter-terrorism colleagues are acquitted of any wrongdoing. The evidence obtained on Hunter is never used, and he is put in the witness protection system. The case against Gates is closed.

A police convoy escorting a civilian under a witness protection scheme is attacked. The witness is hospitalised and all police officers are killed – with the exception of Detective Inspector Lindsay Denton, who organised the operation, and only informed DCC Mike Dryden. Superintendent Ted Hastings asks DS Steve Arnott and DC Kate Fleming to investigate Denton, but Fleming asks not to take part. She explains that she trained together with DS Jayne Akers, one of the dead officers, but she does not disclose that she has also been having an affair with Akers' husband, Richard. She is replaced on the case by DC Georgia Trotman, an ambitious but less-experienced officer, who is troubled that the team are treating Denton as a suspect and delving into her finances. However, she and Arnott quickly grow close and begin a sexual relationship, while Hastings tries to keep up appearances with his estranged wife. Denton, shunned by her fellow officers, is transferred to a missing persons unit and Fleming, undercover once again, becomes her assistant.

In a parallel sub-plot, Denton has a dispute with her alcoholic neighbour. One evening Denton snaps and strikes her with a wine bottle, which she then wipes down and disposes of. When the neighbour reports the attack, Denton flatly denies it.

Fleming follows Denton to the nursing home where her mother lives and sees Denton ring the hospital where the injured witness is being treated. Trotman and Arnott race to the hospital and confront a man disguised as a nurse. Arnott is knocked unconscious, Trotman is thrown out of a window to her death and the witness is killed.

Arnott discovers that a nurse on the witness's ward, Claire Tindall, was threatened into giving the killer access by a man she knows as Joe, whom she is unable to identify. Denton begins her work in missing persons with the case of a vanished 15-year old, Carly Kirk. Denton's neighbour's music again disturbs Denton, who tries to reason with her to keep her music lower at night. The neighbour demands £100 to keep quiet, but Denton gives her only £10. Later, Denton finds her neighbour's door open and the woman asleep, while a chip pan is heating up. On reflection, Denton turns the heat off, takes the money and leaves. As they work together, Fleming attempts to gain Denton's confidence, but Denton sees through this, assaults Fleming and steals her phone. Denton is interviewed by AC-12 and claims that she called the hospital to ask that the witness exonerate her, but the team do not believe her and Hastings arrests her for conspiracy. At this point Denton turns the tables and gives evidence, on the record, that Arnott made an unofficial romantic visit to Claire, Hastings has financial problems (something he previously cited as making Denton vulnerable to bribery) and Fleming's mobile phone has an "interesting" call history (alluding to her affair with Richard Akers). Meanwhile, a story surfaces in the press that DCC Dryden took penalty points for his wife when she committed a driving offence.

Denton is remanded in custody, where she is victimised by both staff and prisoners, having her hands badly burnt and warned not to speak to the authorities. Hastings requests witness protection records from Dryden, allowing AC-12 to discover the identity of the dead witness: Tommy Hunter, the gang leader previously arrested by DCI Tony Gates. Fleming visits Denton, who knows about Fleming's affair with Akers' husband, which Cottan also suspects. Denton's accusations cause friction between Hastings and Arnott. Meanwhile, Dryden's driving offence continues to occupy the press. To draw attention away from himself, Dryden leaks the information that Denton is the officer under investigation. Arnott begins a relationship with DS Nicola Rogerson of Major Violent Crimes, who promises to pass him information. At the site where Denton stole Fleming's phone and blew her cover, Fleming discovers a body, assumed to be that of the missing girl, Kirk. She re-interviews Denton, who claims that Dryden, a married man, had an affair with her and has set her up. Fleming discovers that Dryden and Denton did previously work together, lending some credence to this story. Cottan discovers from forensic accountants that Jayne Akers received a large sum of cash shortly before she died. Arnott visits Denton in prison and says, "I believe you."

Arnott and Fleming persuade Hastings to bring Dryden in for questioning, but find the DCC uncooperative and frosty. AC-12 face a further problem when the Major Violent Crimes team bring in Richard Akers before they can do so, as Arnott mistakenly assumed Rogerson would tell him if they were going to and that Akers was unaware of his wife's corruption and decided he was not a priority. When they do get to interview Akers, he reveals that Jayne kept recordings of Hunter as leverage and stored them at a PO Box. The recordings show Hunter threatening to inform on various parties, including corrupt police officers "from the two-faced bastard right down to The Caddy", unless they ensure his witness protection and immunity are maintained. A recording of Dryden publicly criticising the immunity Hunter and others like him receive suggest he is the corrupt officer in question, and AC-12 begin seeking the identity of 'The Caddy'.

Denton is granted permission to visit her dying mother at her nursing home, but on the return journey, Denton's prison van is run off the road. Fleeing for her life, Denton finds herself confronted by two corrupt police officers, DS Manish Prasad and Trotman's killer, DC Jeremy Cole.

Prasad and Cole torture Denton in a parking garage to find out what she has told investigators, and she reveals only that Dryden set her up. Prasad kills Cole for being "a liability", before Denton manages to escape from her kidnappers. Using the car, she pins Prasad to the wall and forces him to record a confession implicating Dryden before calling Arnott. She escapes on foot and goes to her mother's bedside, where Arnott allows her some quiet time before she is returned to custody. AC-12 find themselves forced to decide where their loyalties lie between Denton and Dryden. With the evidence mounting against the latter, the discovery of pictures showing Dryden and Kirk in a compromising situation is the final nail in the coffin, resulting in Arnott and Fleming finally arresting the DCC. However, when presented with AC-12's evidence, Dryden still strongly maintains he is being set up. Meanwhile, Cottan approaches DC Nigel Morton from his old team. Morton is the officer who sold the story about Dryden's driving offence to the papers, and Cottan blackmails him into falsely saying Cole, the deceased kidnapper, was nicknamed "The Caddy" by fellow officers.

Dryden claims that Denton was at the car park when he was photographed in the car with Carly Kirk. Arnott gets closer to Denton and tells Fleming that he is, as she was earlier, undercover. Arnott finds the pay-off money hidden among the possessions Denton took from her mother's room.

In flashback, events prior to the ambush: Denton follows Dryden to the reception, where she sees Kirk flirting with him. After talking to her in the ladies' cloakroom, Denton follows Kirk and Dryden to the car park, where she sees Kirk fellate Dryden and sees Hunter assault her after she leaves Dryden's car. Denton uses a colleague's computer to report the assault. She later approaches Hunter but DS Akers intervenes. Akers visits Denton at home to request help in saving Carly Kirk by handing Hunter over to his criminal associates, who fear he will inform on them under witness protection. With a pay-off, and appealing to Denton's desire to protect Kirk, Akers persuades her to assist in the handover, which also involves Cottan. The ambush on the convoy is made possible by a tracker on Denton's vehicle. Akers and Hunter are killed to protect Cottan's identity, and Denton is left alive to take the blame, but moves the tracker to the burning vehicle.

The buried body turns out not to be that of Kirk, who has fled the country. Prasad, who carried out the ambush and kidnapped Denton, gives evidence against her in return for reduced sentencing. Morton and Cottan come to an arrangement whereby Morton does not reveal his knowledge of Cottan's activities and Cottan keeps quiet about Morton selling confidential information to the press. Dryden receives a suspended sentence for perverting the course of justice regarding his wife's speeding ticket and resigns from the police force. Denton is convicted of conspiracy to commit murder and given a life sentence. Cottan is asked to remain in AC-12. Major Violent Crimes continue to investigate who organised Hunter's murder, but no police officers beyond Denton are suspected.

Sergeant Danny Waldron and his team of armed officers are deployed to detain Ronan Murphy, a suspect wanted in connection with a plot to commit a gangland execution. After Murphy's car is forced to stop, the suspect runs into a nearby housing estate, where Waldron corners him before the rest of the team arrive. Murphy surrenders his gun, but Waldron shoots him and manipulates the scene to make it look as though Murphy opened fire first. Waldron orders his team to discharge their weapons so as to avoid any trouble. Under interrogation from AC-12, Waldron remains cool until DS Steve Arnott's line of questioning into his private life begins to spook him. When Arnott confronts him outside, Waldron implies that he knows he will not get away with what he has done.

Supt Ted Hastings agrees to send DC Kate Fleming undercover as a new member of Waldron's squad in an attempt to uncover the truth. Waldron's team are tense as a result of being bullied into collaborating with Waldron, and Fleming identifies PC Harinderpal "Hari" Bains as the weakest link, but Bains refuses to testify against Waldron. After pressure from AC-12 department lawyer Gill Biggeloe, Waldron's team are returned to active duty. On their first call, they are tasked with raiding the home of a drug-dealer. Waldron pockets an illegal firearm, then stops Fleming just in time from accidentally shooting at a child who has been left in the house. Waldron demands Fleming transfer out of his squad.

Waldron keeps watch on Bains' house, and later spies on Ronan Murphy's funeral. He follows Linus Murphy, uncle of Ronan, home from the funeral, and holds him at gunpoint, with words that reveal that Ronan and Linus were part of a group who harmed him as a child. He admits to killing Ronan for revenge, and begins to torture Linus. Back at his flat, Waldron leaves a list of names in an envelope addressed to Arnott. When Waldron's team raids another drug house, he orders Fleming to stay downstairs while the rest of the team search upstairs. Fleming hears a shot, and rushes upstairs to find the other squad members standing around Waldron, who has been shot in the neck and is bleeding to death.

Fleming attempts to provide first aid to the wounded Waldron, who dies in the ambulance. The remaining members of the team, PC Rod Kennedy, PC Hari Bains and PC Jackie Brickford, are interviewed by AC-12 and claim Waldron shot himself despite their attempts to stop him. Hastings attempts to arrest them, but Biggeloe convinces him to rescind the arrest. The police search Waldron's flat, finding Linus Murphy's dog (whom Waldron took with him after the murder to help lead the police to Murphy's body). Cottan discovers a box containing the stolen firearm, a photograph of a young Waldron and other boys alongside Ronan and Linus Murphy, and the envelope, addressed to Arnott, containing Waldron's list. Cottan destroys it.

Arnott agrees to take over the investigation of Waldron while Cottan focuses on his colleagues. Arnott locates Linus Murphy's home and finds his tortured and decapitated body, then finds the head in a storage room rented by Waldron. Waldron's team-mates find themselves suspected by their colleagues, and Fleming pressures them further by suggesting she knows they murdered Waldron, indicating she will tell AC-12 about a radio call Bains made to Waldron moments before his death, which was not included in the reports. Meanwhile, former DI Lindsay Denton has appealed against her conviction for conspiracy to murder Hunter, and Arnott is called as a witness. He denies having a sexual relationship with Denton, but the suggestion puts strains on his relationship with his girlfriend, DS Sam Railstone. Scared by pressure from Fleming, Kennedy demands to meet Bains in a disused warehouse, blaming Bains for their situation. This results in a brief physical confrontation. Denton claims in court that she had sex with Arnott.

Next morning, Kennedy's body is found hanged in the warehouse where he confronted Bains. Brickford and Bains meet, and appear the next morning to alter their statements to AC-12. They now claim Kennedy shot Waldron, and Brickford confesses she had a relationship with Kennedy until some months earlier, when they broke up after she had a one-night stand with Waldron. Bains receives a package containing a burner phone, and receives a call from someone who congratulates him on his deception.

Hastings authorises Fleming to begin surveillance of Bains. She watches him meeting with Brickford; when Arnott interviews Bains' superior, Bains panics and is observed calling his contact from a payphone. Fleming asks Cottan to organise a second examination of Kennedy's body, which Cottan intentionally neglects to do. The photo of Waldron and Ronan Murphy is traced to his time in Sands View boys' home. Arnott identifies Joseph Nash, another boy in the picture, and interviews him. Nash explains Linus Murphy was the caretaker at Sands View, that both men abused boys at the home, and that boys at the home were sexually abused by other men both at Sands View and at private parties. He identifies then-Councillor Dale Roach, but Roach is unable to stand trial, due to a stroke.

Brickford visits Fleming to find out what AC-12 has been asking her. Fleming reveals Bains has been making phone calls, seemingly to Brickford. Brickford confronts Bains, who cannot explain the calls. Brickford returns to AC-12 and confesses Waldron was shot in a struggle that Brickford and Kennedy were involved with. Only Bains saw how it began. Bains claimed Waldron had aimed at him, and that Bains grabbed the gun in self-defence. Brickford also confesses to lying about Ronan Murphy's death. Hastings has Brickford's police contract terminated and warns her she may yet be charged. Meanwhile, Arnott orders a second post-mortem on Kennedy's body. It indicates he was murdered. AC-12 moves to arrest Bains, with the assistance of a firearms unit, but his contact, Cottan, warns him and he escapes. He travels to the warehouse where Kennedy was hanged, and finds Cottan. As the firearms team approach, Cottan handcuffs and injures himself to fake a fight. Bains is arrested and the police find a noose, supporting Cottan's story that Bains was attempting to hang him.

At the conclusion of Denton's trial, the jury finds her guilty of perverting the course of justice but not guilty of conspiracy to murder. She is released with time already served. She forgives Arnott outside the courthouse. Cottan is congratulated by his colleagues, but the mobile phones in Bains' home suggest he was acting on orders, and, as Cottan watches the TV news footage of Denton's release, a number of mobile phones in his house begin to ring simultaneously.

Denton is taken to AC-12, where she demands a formal apology. She reveals she has a recording of herself and Arnott in a compromising situation that she will use against him as evidence of perjury if they do not comply with her requests. Denton is forced to work as a supermarket cleaner and later records an exploitative parole sponsor trying to pressure her into sex. Arnott, who is losing confidence within the department, meets with Denton and asks for help in finding the link between Sands View and Hunter.

Cottan is awarded with a commendation for his perceived bravery in arresting a corrupt police officer, Bains. He then proceeds to tell Biggeloe and Fleming he is concerned about Arnott's behaviour and that Arnott is carrying a firearm in his jacket. Bains confesses to killing Waldron on the demands of his unknown contact, but insists he did not murder Kennedy. Arnott and Fleming confront Hastings, saying they believe they should question a retired officer on the subject of allegedly reporting to a mother that a sexual-abuse complaint of Roach was dealt with, resulting in Roach being charged and sent to prison. The mother saw Roach in the newspaper some time later, so knew she had been lied to. The death of a former social worker at Sands View, ruled as a suicide, is reinvestigated due to allegations that he attempted to report the abuse to police and may have been murdered to prevent his case being pursued. They eventually interview former Vice Chief Superintendent Fairbank, who claims to remember no details of the case. He and Hastings share a handshake that Arnott identifies as Masonic.

Cottan meets former colleague DC Nigel Morton and threatens to derail his pension if he does not hand over the last known evidence of Cottan's true allegiance – a mobile phone and SIM card with records of calls made to Hunter. Morton tells AC-12 'The Caddy' is probably still alive, leading Cottan to create a presumptive profile which he deliberately engineers to fit Arnott. Morton hands the phone and SIM to Cottan, who then destroys, in a car park (where Cottan was prepared to kill him had he had not complied), but it is revealed it was a duplicate and Morton still possessed the items in his car.

Cottan continues to attack Arnott by pressurising PC Maneet Bindra to hold off on key forensics on the envelope found in Waldron's flat and then by spreading rumours surrounding Arnott's conduct, including telling Fleming he has a lead linking Arnott to Denton's bribe money. Arnott is eventually suspended by a hesitant Hastings, to whom he reveals the redacted file that would have linked Ronan and Linus Murphy to Hunter. Hastings challenges Biggeloe on this but is rebuffed. Meanwhile, Fairbank is called in for interview and maintains his ignorance as he is presented with images of alleged abusers. Nash is shown a photograph of Fairbank, ostensibly to identify him as an officer that had overlooked accusations, but his reaction suggests Fairbank was one of the abusers.

Arnott and Denton investigate the possibility that Waldron hid another copy of his list of abusers. Denton surmises the list is kept digitally, so she tricks Arnott into pursuing a false lead while she, having stolen Arnott's pocketbook, goes to an internet cafe to access Waldron's emails. She finds a photograph of the list in his drafts folder and transfers it to hers as insurance.

Cottan is outside and summons her to his car - revealed to be Arnott's service vehicle with false number plates - and drives her to an industrial site where he offers money for the information at gunpoint. Denton, staying true to her morals as a former police officer, rejects this and defiantly emails the list to Hastings, for which Cottan murders her.

Fairbank is arrested and interviewed on new information submitted by Nash during an identity parade. During the interview, Hastings receives the list sent by Denton which identifies Fairbank as one of the abusers. Meanwhile, Arnott awakens to find his pocketbook missing and his car stolen. As he reports the car missing, Hastings attends a call to a dockers' yard where the body of Denton has been discovered in Arnott's car. Arnott is arrested on suspicion of murder. During questioning, evidence identifying Arnott as 'The Caddy', such as money from Denton's bribe and the mobile phone he used to contact Bains, are shown to have been found in Arnott's gym bag. Arnott realises the evidence records relating to his signing out a firearm have been doctored to suggest he did not return it. He knows somebody within the department has set him up, but has no way of proving it. Cottan reveals that he has suppressed evidence regarding the envelope left by Waldron.

With authorisation from the East Midlands AC-3 superintendent, Fleming has been carrying out an undercover operation into the officers of AC-12. She highlights inconsistencies within Cottan's line of questioning against Arnott. Having spoken with DS Nicola Rogerson, she identifies Cottan was the one who fed AC-12 the doctored file on Ronan Murphy. Hastings confronts Biggeloe, who confirms it was Cottan who gave her the file. Bindra later reveals to Fleming it was Cottan who told her not to inform Arnott of the forensic findings of the envelope found in Waldron's flat. Fleming uncovers CCTV which confirms Arnott's car was driven on false number plates shortly before the time of the murder.

Cottan denies all knowledge of the doctored file and claims Arnott must have been the one responsible, having previously had a relationship with Rogerson. Hastings pressures Cottan on the suppression of the golf tee from evidence, as well as the fact that no blood was found on the tee despite blood being found on both the envelope and the list. Cottan claims he only removed it to prevent Arnott from becoming aware he was investigating him. Fleming reveals new forensic evidence against Cottan - fibres from the rope found at Kennedy's murder scene, in the boot of his car. Cottan strenuously denies any involvement with Kennedy's murder. Hastings probes him on evidence submitted by Morton, which suggests he pressured Morton into wrongly identifying 'The Caddy', and also used a secret mobile phone. Phone records reveal he previously called Hunter several times in the months leading up to his death. Fleming presses Cottan on the lead he claimed to have on Arnott linking him to Denton's bribe, which Hastings points out could not have been the money in Arnott's gym bag, as Cottan had made that claim the night before the money was discovered following Denton's murder.

Questioned about his whereabouts on the morning of Denton's murder, Cottan states that he was at home, but Fleming calls his bluff by implying that she might have visited the flat then and asks him to reaffirm his testimony. Cottan sends a text: "Urgent exit required", to three recipients, 'Removals', 'Talk', and 'Ride'. An armed officer guarding the interview shoots his colleague dead, then fires shots into the interview room, enabling Cottan to escape, and hands Cottan his pistol. Fleming takes the shot officer's weapon and deals with the other armed officer. She catches up with Cottan and dismisses the suggestion that they might have had a sexual relationship, telling him that all along she has been carrying out surveillance on him.

Cottan is rescued by a getaway car which unsuccessfully attempts to immobilise Fleming, who manages to shoot the driver and brings the car to a standstill. As she goes to confront Cottan, the driver fires three shots aimed at her, but Cottan jumps in front of her, taking the bullets. Cottan records a dying declaration that directly leads to Fairbank being imprisoned for ten years. The epilogue reveals Arnott is exonerated and returns to AC-12, where Hastings and Fleming also remain; Fleming receives an award for her bravery and is promoted to Detective Sergeant; Denton receives posthumous recognition for her contributions by the coroner but is buried in a local authority funeral with no mourners; Morton retires on full pension plus disability benefits (although it's known he's faking his condition).

DCI Roseanne "Roz" Huntley identifies and captures a suspected serial killer. The suspect is uncooperative under questioning and appears not to have an alibi. After months of fruitless investigation, ACC Derek Hilton puts pressure on DCI Huntley to charge the suspect. FC Tim Ifield informs DCI Huntley of concerns he has relating to the forensic evidence, which he believes proves the suspect's innocence, but DCI Huntley seems uninterested. Ifield alerts AC-12 to a possible miscarriage of justice. He tells DS Steve Arnott that he believes DCI Huntley is deliberately ignoring forensic evidence which proves the suspect's innocence. Ifield explains the forensics suggest someone has manipulated the crime scene in an attempt to frame the suspect. With the authorisation of Superintendent Ted Hastings, DS Kate Fleming goes undercover to investigate DCI Huntley, and also interviews the suspect. Arnott informs DCI Huntley that AC-12 are investigating. DCI Huntley suspects Ifield and turns up at his house at night, where she accuses Ifield of talking to AC-12. Ifield admits this, and a row breaks out over the guilt of the suspect. DCI Huntley causes Ifield to burn his hand and he slaps her. When she accuses him of assault, he pushes DCI Huntley who hits her head, appearing fatal. Fearing the worst, Ifield departs to a hardware store, purchasing cutting tools in order to dispose of the body. While he's preparing to carve DCI Huntley's head, she wakes up.

When interviewed by Fleming, Michael Farmer, the alleged serial killer, correctly gives the name of the hospital he was in when Leonie Collersdale, the second murder victim, was killed, throwing further doubt on his guilt. DCI Huntley fails to report for duty. A dismembered female body is discovered, identified as Collersdale's. When she reappears, Huntley brushes off her absence, saying she had a bug. Hilton tells Huntley that Hastings is going to serve her with a Regulation 15 notice. AC-12 interviews Huntley and throws doubt on forensic evidence concerning the attack on Hana Reznikova, which Huntley alleges is by Farmer, and says Huntley failed to present any contrary evidence to the Crown Prosecutor. Hastings says he will be recommending she be taken off the case. Huntley is later shown examining Ifield's laptop in her car. Ifield is missing; Arnott goes to his flat, breaks in and finds his body. Three fingers have been amputated. Huntley arrives and takes charge of the crime scene, despite Arnott's protests. Inside, she notices the blood spatter on the edge of the kitchen counter, which is hers. She hears the sample being labelled KRG-13, but mistakenly notes it as KRG-30. Huntley obtains a sample of Ifield's blood from her car, labelling it KRG-30. Managing to gain access to the forensic samples, she cannot find KRG-30, but realises her error and over-writes her sample bag as KRG-13. Ifield's mobile phone records are investigated, and communications with Reznikova are found. Arnott learns from forensics that Ifield was probably wearing a white forensic oversuit at the time of death. Summarising the evidence, he implies to Hastings that Huntley is Ifield's killer. Huntley takes a further sample of Ifield's blood and inserts it into forensic submissions for Collersdale's murder, implicating him. She requests further forensic tests; forensics find the match to Ifield. Fleming passes the Inspector's exam. Hastings informally indicates to Fleming that he has decided on Arnott for promotion. Huntley tells Hilton that, if AC-12's informant was Ifield, the evidence against him undermines his evidence and therefore undermines the case against her. She tells him she's convinced Ifield's murder and Farmer's crimes are connected.

Huntley continually shuts Fleming out of meetings. The officers try to find links between Ifield and Farmer. In order to gain access to information from the investigation, AC-12 seize evidence forcibly, since Fleming is unable to gain access undercover. A security camera shot of Ifield in the balaclava mask and purchasing the cutting tools found at his flat does not convince Arnott that Ifield is 'Balaclava Man'. His requests to Hastings to open a second investigation are authorised. Arnott theorises Huntley was the one to kill Ifield after her phone history proves suspicious. DC Taylor notices Fleming checking the forensics (that Huntley had swapped) and reports back to Huntley. Reznikova is discovered to be a prostitute and admits having slept with Ifield. Arnott begins questioning Huntley's husband, Nick Huntley, regarding her whereabouts on the night Ifield died, but he refuses to confirm or deny his wife's alibi. DC Desford is assigned to assist Arnott with the investigation. Having heard no response from Nick, Arnott returns to his office to question him and misses various voice messages from AC-12 warning him Nick could be dangerous, as his car was identified both approaching and departing from the area near Ifield's flat on the night of his murder. On the way up in the lift, Arnott is attacked by a man in a balaclava, hit in the face with a baseball bat and thrown down the stairs on the third floor.

After the attack on Arnott, Huntley picks up her husband from his office and advises him to go inside away from CCTV and ask security to look for Arnott. Arnott is found to be badly injured, but alive. Nick Huntley is interviewed by Hastings and Desford. He says it was a coincidence that only he knew Arnott was to be at his office building when he fell down three flights of stairs. He did not explain why his car was seen near Ifield's flat when Ifield was murdered. Nick said his wife was asleep in the spare room that night. Nick's solicitor, Jimmy Lakewell, says that with no evidence or CCTV, Nick should be free to go. AC-12 find a CCTV image of a man in a balaclava by Nick's offices at the time of the attack on Arnott. In her car, Nick and Huntley argue, Nick stating he had seen her going into Ifield's flat that night. Huntley says she was having an affair. Quizzed by Hastings in hospital, Arnott is unable to identify his attacker. At a meeting in his club, Huntley appeals to Hilton to help her by making an official complaint against AC-12 and closing the case against Farmer. Hilton says there are some things he could tell Huntley to help her with AC-12. Despite her friendliness, she rejects his advances, but suggests they meet again. Huntley and Fleming talk with Kevin Gill, the forensics officer: KRG. Huntley's wrist injury is now worse; she has it examined by a doctor, who takes a swab. Huntley and her husband argue again, at home. He says he cannot believe anything she says any more and he knows she was in the flats where the murder happened. She slaps him, twice; he breaks down. AC-12 finds an anomaly in the second forensic examination of KRG-13, the blood-spatters sample with the overwritten label: traces of white fibres from a forensic oversuit. Huntley is interviewed by AC-12. After running through evidence, Hastings says he will be recommending she be suspended from duty immediately. Huntley then accuses Hastings of sexism. She has identified Fleming as an undercover AC-12 officer in her team. Huntley asks Hastings whether he is a Mason. All through this altercation, Hastings is asking her where she is getting this information. She suggests Fleming was induced to tamper with evidence concerning Ifield's murder, accuses Hastings of a conspiracy to pervert the course of justice in the case against her, and brandishes a supporting letter from Hilton, recusing AC-12 from their inquiry. Hilton and Huntley meet again at his club, but Huntley again rejects him, saying she loves her husband. Afterwards, PC Maneet Bindra is seen handing a file to Hilton. After getting home, Huntley tells her husband he is no longer a suspect.

Hastings accuses Hilton of a wilful effort to bury allegations of wrongdoing. Hilton denies this and demands a copy of the recording of DI Matthew "Dot" Cottan's dying declaration. Hastings declines, claiming no reciprocity. At a team briefing, DS Twyler reports biometrics suggest the 'Balaclava Man' who attacked Arnott and the 'Balaclava Man' seen in the area of Collersdale's murder are the same man. Huntley says maybe, but Farmer may change his plea to guilty. Twyler has doubts and tells a colleague that Huntley is keen to pin Ifield's murder on Reznikova. Fleming tells Hastings the leak of her cover is likely to have been from within AC-12. More female body parts are discovered: they are more of Collersdale. Arnott arrives at the office in a wheelchair, returning to work despite his limited mobility. He says Huntley cannot be allowed to mislead the investigation and Hilton said nothing about AC-12 not investigating Ifield's murder. He suggests looking at Nick Huntley's alibi, to which Bindra finds it does not clear him. Bindra asks Desford for his username and password, claiming they are being changed, but uses them to enter the police system and download the recording of Cottan's dying declaration, which she passes to Hilton. Bindra then asks to take early pregnancy leave.

Fleming notes the sensitive information access and tells Hastings, who reprimands Desford. Desford says he is being scapegoated, requests a transfer and leaves the office. Arnott visits Farmer in prison, suggesting a change of solicitor; Farmer says he liked the one he had before. Arnott brings Farmer's Nan to the prison to try to prevent him from changing his plea; she succeeds. Arnott tells Fleming the framing of Farmer could not have been random and finds that Farmer's earlier solicitor was Lakewell. Arnott and Fleming visit Lakewell, but get nowhere. Huntley's wrist is beginning to stink; the doctor who took the swab tries to contact her. At home, she faints after taking off the dressing. Nick meets Lakewell, saying Huntley is lying and has a wound which dates from the night of Ifield's murder; he believes she murdered Ifield. Lakewell floats the idea of a voluntary interview and immunity. FC Rupal Pandit reveals proof the latest remains had been frozen and were buried after both Farmer's incarceration and Ifield's murder, and suggests a team briefing to Huntley. Pandit goes to the team office for the briefing, but no-one has been made aware of the information. Twyler, who hears this, meets Fleming and, stressing anonymity, relays the information. This triggers Arnott's memory: Jackie Laverty's body was buried after being kept in deep freeze and men in balaclava masks with baseball bats were involved. He suggests the Laverty case, which is unsolved, be re-opened, giving them the chance to examine the latest forensics. Huntley is in hospital. Her husband explains to her that her hand had to be amputated to save her life.

With the new forensic evidence, Arnott finds the blood sample of Ifield (relating to Leonie Collersdale's murder) was contaminated with white forensic oversuit fibres, just as the blood spatter on the kitchen counter had been; so both were falsified. He says he believes Huntley has been protecting her husband. Hastings says "to hell with ACC Hilton" and instructs Fleming to go and pick up the Huntleys, but DCI Huntley has discharged herself from hospital. Taylor picks Huntley up and tells her Lakewell has requested a voluntary interview with independent detectives about Ifield's murder, seeking immunity. Huntley meets her husband at home. She says they are getting a divorce and accuses him of amputating her hand in retribution for her having an affair. He says it was all her lies and she had MRSA in her wound. She arrests him for Ifield's murder. Once he has been taken away, Huntley reaches for the holdall she had kept with contaminated items from the scene of Ifield's murder and runs his hairbrush over one of her husband's jumpers.

Hastings, Arnott and Fleming visit Hilton. Arnott and Fleming explain evidence was planted in order to falsely implicate Ifield in Collersdale's murder and that Ifield had uncovered a conspiracy to frame Farmer for the murders. Hastings says he would like to arrest DCI Huntley on suspicion of perverting the course of justice. Hilton brushes this aside and reminds them AC-12 is recused, and that these are DCI Huntley's cases. Hastings says they came across this as a result of looking again into the disappearance of Laverty. Hilton looks disturbed by this revelation, then praises their work, but again calls for their files to be handed over to an impartial authority. Hilton tells Arnott and Fleming to leave and plays Hastings the recording of Cottan's dying declaration in which it is revealed that the top corrupt official who promoted Cottan had a name beginning with "H". Hilton tells Hastings, referring to him as "H", that he will be served with a Regulation 15 notice. Fleming drives Arnott home, discovering he is unable to look after himself. He confesses to Fleming he may never walk again; she consoles him. Before he is interviewed, Nick Huntley again tells his solicitor that DCI Huntley is lying about everything.

Nick Huntley is interviewed regarding his movements on the night of Ifield's murder. AC-12 realise it is consistent with the few CCTV recordings and witness testimonies they have, but struggle to find any evidence to implicate DCI Huntley. Desford is transferred to Hilton's team and is in conflict with AC-12. DCI Huntley leads them to her husband's jumper, which allows Polk Avenue to formally charge him with Ifield's murder, as well as the previous crimes attributed to 'Balaclava Man'. Nick Huntley asks Lakewell to get AC-12 involved, but he does not. DCI Huntley asks Taylor to secretly analyse some telecoms. Arnott realises Ifield's killer stole a tracksuit, because his or her clothes would have been covered in blood, and then would have changed clothes again when they got home before hiding both. They narrow their search to a three-hour window shortly before Ifield's body is found, and are able to spot DCI Huntley driving towards a large woodland area. They discover the MRSA that infected DCI Huntley's wound is likely to be traceable to Ifield. Arnott works out that the time taken for DCI Huntley to drive to and from the area would have only given her a few minutes to hide the evidence. They find that she had previously led a search of the area and that she had hidden her own incriminating evidence in the same place.

DCI Huntley is subsequently arrested for Ifield's murder and requests Lakewell as her solicitor. After being interrogated and facing damning evidence against her, DCI Huntley admits to Ifield's accidental death in self-defence and to framing her husband. Just as the interview appears to be concluding, DCI Huntley arrests Lakewell for perverting the course of justice. It is revealed that Lakewell, whom Nick Huntley rang as Arnott was approaching his office, used a burner phone to tip off Hilton, who then contacted the man who attacked Arnott. The burner phone was also used by the man who kidnapped Hana, planted the evidence in the house of Michael Farmer, and attacked Arnott. Lakewell had attempted to frame Farmer, on the orders of the people running a scheme to get vulnerable police staff to manipulate evidence so that they can be pressured later. DCI Huntley proves Hilton's involvement through the number he gave her while attempting to seduce her. Hastings and Fleming head to Hilton's office, but Desford has tipped him off, and he has left by the time they arrive. Desford attempts to leave with Lakewell, but Arnott and DCI Huntley stop him, saying he has probably been set up to be murdered. Hastings and Fleming return but one of the balaclava men (Lakewell admits there are many) holds a security guard at gunpoint. Hastings promptly shoots him. When they get upstairs, Desford has pulled a gun on Arnott. However, Huntley is able to persuade him to surrender it. Hilton is found dead the next day, his death is ruled a suicide, but took place at the same location as a murder relating to the Sands View child molestation ring. Hastings, seemingly no closer to untangling the bigger conspiracy, admits he thinks this has become a lifetime's job. The biometric data of the dead balaclava man is revealed to match the data from photographs and footprint of the balaclava suspect.

The epilogue reveals DCI Huntley was sentenced to ten years for Ifield's manslaughter and the subsequent cover-up. She and Nick remain married. Lakewell pleaded guilty to perverting the course of justice, but declined witness protection and refused to testify. Farmer was released without charge, and the Regulation 15 order against Hastings was dropped. Hastings remains in charge of AC-12.

An organised crime group (OCG) hijacks a police convoy transporting seized drugs and kills three armed officers; AC-12 suspect that undercover police may be involved. Sergeant Jane Cafferty survives the attack, drawing suspicion. PC Maneet Bindra is interviewed by Superintendent Hastings, DS Steve Arnott and DI Kate Fleming, due to her involvement with her cousin, Vihaan Malhotra, in spying on AC-12 and leaking information to the organised crime group for ACC Derek Hilton. In a further attempt to protect Malhotra, she contacts the organised crime group. Corbett accuses her of infiltration and she is killed. AC-12 discover that the undercover officer (UCO) is DS John Corbett and not, as originally assumed, Lisa McQueen.

Lisa McQueen blags her way past a police guard and Cafferty's husband to talk to her in private. Another police leak enables the OCG to carry out a raid led by Corbett on a weapons convoy. Cafferty is found with £5,000 in a brown envelope by Arnott in his attempts to gather information. Corbett convinces him he is "straight". Hastings is approached by Mark Moffatt regarding his failed property investment. Moffat hands him a big brown envelope. Roisin Hastings insists on their divorce. From photographs, Cafferty identifies who recruited her.

Arnott tells Fleming and Hastings of his liaison with Corbett. After their initial shock, they agree to continue with Arnott's proposal. AC-12 identify that the organised crime group are involved in sex trafficking at a block of flats. AC-12 raid the flats and the group's front print shop. Corbett tips Arnott off about a larger-scale raid on the Eastfield police depot and states that a corrupt senior police officer will be there. Arnott is drawn away from the raid by a false 'Status Zero' call. Corbett shoots the senior police officer, who arrives with tracker information, and is revealed to be DCS Hargreaves. Corbett, on learning of his death, flees. Using false AC-12 ID, he enters Roisin Hastings's flat.

AC-12 discover that Corbett has links to Northern Ireland. Arnott arranges to meet Corbett, backed up by armed officers. Corbett tells Arnott that he had attacked Hastings's wife Roisin who is now in hospital. Hastings orders Arnott via his wire to shoot Corbett, but he refuses. Corbett reveals the location of a meeting between the OCG and "H" and then escapes. AC-12 and the Cybercrime unit impersonate "H" and make contact with the OCG via a laptop. The OCG transport a group of women to their base to be trafficked. Corbett wants to free the women and thinks that McQueen will support him in this. However, the OCG kill Corbett as they realise that he is the rat.

The OCG dump the bodies of Corbett and Jackie Laverty which are later found by police. AC-12's investigation into Operation Pear Tree is suspended. Hastings continues to make contact with the OCG. He persuades McQueen and Miroslav Minkowicz that he can handle the goods from the raid, until armed officers catch up with Hastings. Minkowicz is killed and McQueen is arrested. AC-12 discover that Hastings visited Lee Banks in prison, leading Fleming to raise concerns with senior officers. Hastings is suspended and investigated by DCS Patricia Carmichael, and is subsequently charged with conspiracy to murder Corbett.

Carmichael and her team continue to interview Hastings. They question him about the £50,000 in banknotes they found in his hotel room. Gill Biggeloe, who sits in on the interview, identifies errors in the police search procedure of Hastings's hotel room. This leads Carmichael's team to interrogate Hastings's communications data for further evidence. Meanwhile, Fleming and Arnott revisit Corbett's widow and DSU Powell. They discover that Biggeloe was involved in selecting Corbett for Operation Pear Tree, and specifically identified Hastings as someone to pursue. Fleming and Arnott present their findings to Hastings. Biggeloe was recruited by the OCG while she was working as a defence solicitor. She texts the OCG for her escape but to no avail. One of Carmichael's team, PS Tina Tranter, attempts to stab Biggeloe but is shot by Arnott.

The epilogue reveals that Operation Pear Tree is now officially closed. Fleming received a commendation for her investigation into Pear Tree. Arnott was commended alongside her after an independent panel cleared him in shooting Tranter. McQueen received immunity in exchange for providing information about the OCG, and now volunteers as a youth worker. Ryan Pilkington is now a student police officer. Biggeloe received immunity and a new identity. Hastings received a final written warning, but continues to lead AC-12. Moffat, standing trial, claims that the £50,000 found in Hastings's hotel room is only half of the sum that was handed to him. The remaining £50,000 is never found, but Hastings is seen approaching Corbett's widow with an envelope.

Based at Hillside Lane police station, DCI Joanne Davidson is the Senior Investigating Officer (SIO) of Operation Lighthouse, the investigation into the murder of journalist Gail Vella. Fleming is now in her team. They are tipped off by a covert human intelligence source (CHIS) that someone using the alias Ross Turner had boasted of killing Vella and are given his address. Buckells orders them to wait until the morning. Davidson, whilst in a convoy of armed police speeding to the address, orders it to intercept an apparent hold-up at a betting shop. One suspect is shot dead and the others arrested; the convoy is delayed but Terry Boyle, a man with Down syndrome, who has previously been abused and coerced into helping the organised crime group in earlier series, is eventually arrested. PS Farida Jatri reports her concerns about Davidson to AC-12's DS Steve Arnott, who covertly begins looking into Davidson's conduct with regard to the betting shop hold-up. Neither her team nor Arnott is aware of Davidson's affair with PS Jatri, with whom she has just had a messy break-up. Two sets of fingerprints are found in both Turner's flat and in Boyle's home: those of Boyle and those of Carl Banks, a known criminal. The body of the missing CHIS is discovered. When Hastings gives Arnott the order to investigate Davidson, Arnott goes to inform Fleming, and finishes by saying, "Either you're our CHIS or you're embedded as a UCO (undercover officer). That's why I'm giving you a heads-up, Kate." She does not commit herself.

Arnott and Bishop learn from Nadaraja, Gail Vella's TV producer, that she was developing a podcast about high-level police corruption, and that her home had been ransacked - details Davidson seemingly overlooked - which might indicate an OCG hit. PS Jatri requests a transfer for personal reasons. Her replacement is PC Ryan Pilkington. Carl Banks's body is found in a condition reminiscent of an OCG execution, with a knife nearby containing the dead CHIS's fingerprints. Hastings promotes Arnott to temporary DI. AC-12 raid Hillside Lane to seize evidence for their investigation, but are thwarted when Davidson reveals she put in a request to restrict access to Operation Lighthouse documents, causing a 24-hour delay in the investigation. Arnott and Hastings realise that Fleming tipped Davidson off. After Arnott sees Hastings with Steph Corbett, he drives to her house. She witnesses his suffering and his addiction to painkillers. Davidson is summoned to an AC-12 interview. As she is placed under arrest, Davidson suggests that AC-12 should investigate all who have had access to the CHIS's information. PS Jatri is sent to prison when a search of her house reveals a stash of burner phones. She tearfully insists that she is being framed and reveals her personal relationship with Davidson. Davidson is released; she drives herself to take delivery of another burner phone. Alone in her car, she is distraught.

After a witness identifies him as having argued with the deceased CHIS regarding Vella's murder, Boyle is brought in for further questioning. He comes close to sharing new information about a man who "did [the murder]", but Davidson halts the interview. Pilkington, accompanying PC Lisa Patel who is driving Boyle home, diverts to a different route. He forces the car into a reservoir, drowns Patel, and attempts to drown Boyle. Fleming had been tailing them and arrives in time to thwart him. She notices a window in the vehicle where Pilkington was sitting is open, and reports her findings to AC-12. Arnott recognises Pilkington and Fleming remembers interviewing him as a juvenile, in connection with an old AC-12 case. Steph tells Hastings of Arnott's painkiller addiction; Arnott makes an unsuccessful attempt to get clean. He spends a platonic night with Steph. When she leaves to go to work, he searches her house; discovering a large amount of money in the loft. After Boyle's freezer is found in a dump, samples of Jackie Laverty's blood are found inside. In Brentiss Prison, Jatri is prevented from attending an AC-12 interview when an OCG warder breaks her wrist. DSU Buckells and Deborah Devereux, the witness who named Boyle, have had a sexual relationship. Buckells is handed over to AC-12. At home, Davidson messages an unknown individual on a laptop that everything has been taken care of.

Buckells is interviewed by AC-12 and charged. Wise informs Hastings she intends to merge AC-12, AC-3 and AC-9 to form one unit and to reduce the staff by 90% and that he should take retirement to avoid further disciplinary action. A betting shop robber is interviewed by Lomax and Fleming in gathering more information about firearms. As forensic analysis at Jatri's home is completed, Davidson watches before sending another message through her laptop, "I'm finished". Arnott asks DI Rogerson to hold off. He deletes emails from Occupational Health about discussing his drug test. Arnott recognises the voice of jailed solicitor Jimmy Lakewell on a recording obtained from Vella's office. Davidson tells Pilkington she will complete his commendation and then discharge him from her team. When Arnott and Bishop interview Lakewell in prison, he is afraid to talk. In moving him into secure custody, the OCG ambush the convoy. In the shootout, PS Ruby Jones is killed by a sniper, whom Arnott then kills with his handgun. Pilkington explains to Davidson why he will not be removed from Operation Lighthouse. Lakewell is murdered by OCG prisoner Lee Banks in his prison cell as Buckells watches. Forensic results from Jatri's house reveal Davidson's fingerprints were found, but that a DNA test found a match to a nominal located on other police databases.

Arnott and Bishop examine the cell where Lakewell was found "hanged", and are frustrated at having no CCTV footage. Arnott tells Bishop to look into Lawrence Christopher, the case that Vella was investigating before her death. Lomax debriefs Fleming and Davidson on the potential locations where workshopped firearms may have been made. Fleming meets AC-12 and is shocked to learn that Davidson has a partial DNA match to Tommy Hunter, the results indicating this arose from incest. The team watch a press briefing by Osborne, after which Hastings learns from Sindwhani that he should continue to investigate the situation before his retirement. Arnott discovers that the missing £50k from their last case was given to Steph Corbett by Hastings, which he reveals to Fleming. MIT run a sting operation against the sites investigated by Fleming, only to find that the first is empty. Pilkington is observed by AC-12 using a concealed phone to contact the OCG, two members of which attempt to remove the machinery, but are killed by AC-12 AFOs. Fleming is informed of the actions of Pilkington but no arrest is made. Bishop explains to Hastings and Arnott about the Christopher case, leading Arnott to discover that Thurwell was involved in a previous AC-12 case about child sexual exploitation at Sands View, which was covered up. Davidson attempts to force Fleming to submit a transfer request but she refuses, annoying Davidson who is under orders to "get rid of her". Arnott tries questioning Fairbank but gets nowhere; back at the office, Bishop tells him that Vella tried to interview many of AC-12's former adversaries such as Manish Prasad, Hari Bains and Roz Huntley, but only Lee Banks had agreed to talk. Arnott talks to Banks and learns of Hastings's involvement in "tipping off" the OCG about a rat in their team. Davidson lures Fleming to a disused industrial site, but Fleming also tells Arnott. He is delayed due to the arrival of DCS Patricia Carmichael; while AC-12 are en route, Fleming and Pilkington have a stand off with weapons drawn. Two shots are fired.

AC-12 arrive at the industrial estate to find Pilkington's body, with two gunshot wounds to the chest. Having fled the scene, Fleming and Davidson go to Arnott's home and take his car. Davidson shows Fleming the shuttered print shop opposite Boyle's flat, and tells her to search it. They are immediately ambushed by police vehicles, and reluctantly surrender. Carmichael assumes control of AC-12, and reveals to Hastings that they were detected so quickly because AC-12 vehicles have tracking devices, as neither she nor Chief Constable Osborne trust them. Davidson is interviewed first, and although she is shaken to learn the truth of her parentage, she initially gives 'no comment' answers. She also covers for Fleming by lying that she had shot Pilkington whilst disarming her. As questioning continues, Davidson shares details of being coerced by the OCG, although Carmichael repeatedly tries to steer away from this subject. Carmichael terminates the interview, and Davidson is charged. Fleming is released, now the most senior officer in the MIT, and she and Arnott order the floor to the weapons workshop to be excavated. Bishop calls them back to AC-12, where police in Spain are raiding Thurwell's suspected residence. They find two decomposed bodies. In prison, as Davidson is being taken to her cell, she is confronted by two OCG prison guards, who note the two CCTV cameras monitoring the entrance to her cell and retreat.

A lockbox is discovered beneath the floor of the workshop, containing the weapons used to kill Vella, Laverty and former AC-12 officer Maneet Bindra. Arnott finally meets Occupational Health. Davidson is unexpectedly taken to a prison transport van, seemingly for further interview. An order for her elimination from the "Fourth Man" is obtained by AC-12, who intercept the van and arrest the OCG hitmen and prison guards. Davidson is offered witness protection, and names Fairbank as her believed father. The Fourth Man is finally identified as Buckells, who declares that he will take immunity and witness protection, and admits that he has been the OCG's point man for years, passing on orders to other corrupt officers. However, he is informed that he will be ineligible for either immunity or witness protection, and is arrested. After being confronted by his team, Hastings confesses to giving bribe money to Steph Corbett. He tells Carmichael that it was due to wilful institutional blindness that Buckells was able to be promoted so far, despite his apparent incompetence and actual corruption. He also confesses to accidentally tipping off the OCG about Corbett.

In an epilogue, Boyle is rehoused, charges against Jatri are dropped, and Davidson begins a new life in an undisclosed location with a new girlfriend. Darren Hunter is investigated for his part in the murder of Christopher. Osborne seeks public interest immunity for proceedings against Buckells, in order to prevent evidence of institutional corruption from being heard in court, and his structural overhaul proceeds, leaving AC-12 at its weakest.