Line of Duty (series 4)

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.

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.