Series 4 consists of six episodes and originally aired on BBC One between 26 March 2017 and 30 April 2017.
- Thandiwe Newton as DCI Roseanne 'Roz' Huntley
- Martin Compston as DS Steve Arnott
- Vicky McClure as DS Kate Fleming
- Adrian Dunbar as Supt. Edward 'Ted' Hastings
- Jason Watkins as FC Timothy 'Tim' Ifield
- Lee Ingleby as Nicholas 'Nick' Huntley
- Paul Higgins as ACC Derek Hilton
- Gaite Jansen as Hana Reznikova
- Maya Sondhi as PC Maneet Bindra
- Royce Pierreson as DC Jamie Desford
- Claudia Jessie as DC Jodie Taylor
- Mark Stobbart as DS Neil Twyler
- Anneika Rose as PC Farida Jatri
- Nigel Boyle as DCI Ian Buckells
- Aiysha Hart as DS Samantha 'Sam' Railston
- Tony Pitts as DCS Lester 'Les' Hargreaves
- Patrick Baladi as James 'Jimmy' Lakewell
In series 4, Thandiwe Newton join the cast as DCI Roz Huntley, who came under suspicion of falsifying forensic evidence.
After months of investigation into a balaclava-clad serial killer, DCI Roz Huntley became an AC-12 suspect when her colleague, Forensic Coordinator Tim Ifield, alerted the anti-corruption unit to the fact she had deliberately ignored forensic evidence in order to press charges against a suspect, who he believed to be innocent.
DS Kate Fleming was placed undercover on Huntley’s team as DS Kate Flynn as AC-12 began their investigation. But after being paid a visit from DS Steve Arnott, Huntley immediately suspected Ifield of ratting her out to AC-12 and paid a visit to his home.
After a physical altercation ensued, Huntley ended up presumed dead. However, it was later revealed that she survived Ifield’s attack, and then killed him herself. She later covered her tracks by deep cleaning the flat and cutting off three of his fingers that had scratched her on the arm. This was an injury continued to worsen as the series went on, and would prove to be vital in nailing her.
When the alarm was raised over Ifield’s disappearance, Roz arrived at the scene and heard that a sample of blood has been taken from a spatter on the kitchen counter where she’d hit her head during her showdown with Ifield. Knowing this was her blood, she then deliberately labelled the sample incorrectly.
After phone calls between Ifield and one of balaclava man’s spared victims (Hana Reznikova) were found discovered in his call history, Huntley then sought to link him to the case she was building against the original suspect, Michael Farmer. She did by falsifying forensics with samples of Ifield’s blood she had from the white forensic oversuit that he had been wearing when she killed him, although she had taken him out of this and removed it from the crime scene.
AC-12 later discovered footage of a balaclava-clad Ifield at a DIY store purchasing cutting tools (which he actually bought to cut up Huntley’s body, believing he’d killed her) but it did not convince Arnott of his guilt.
After Huntley’s husband Nick was questioned, his car was then picked up on CCTV Ifield’s flat on the night of his murder. Arnott headed to his offices, but upon his arrival, was left for dead after being pushed down a flight of stairs by a man in a balaclava. While Arnott ended up surviving the attack, he was left in a wheelchair.
Nick Huntley was later interviewed by Superintendent Hastings over Arnott’s attack, but let go at the insistence of his solicitor, Jimmy Lakewell. Nick later argued with his wife, who he accused of having an affair with Ifield, having seen her go into his building on the night of his murder using his car.
With the net tightening on her, Huntley appealed to ACC Derek Hilton for help, who gave her information about AC-12 to use against them, in an attempt to stop their investigation. During their meeting, he also came on to her, giving her a phone number she hadn’t been aware of before.
When the blood sample Huntley doctored returned from the lab at AC-12′s request, it was revealed to contained white fibres from the forensic oversuit. The team brought her in for questioning, believing they had enough evidence to suspend her from duty. However, she played the trump card Hilton had given her, revealing she knew Fleming was an undercover officer, and accused AC-12 of planting her in order to tamper with the forensics. With the support of Hilton, she managed to get the investigation shut down... at least so she thought.
Meanwhile, AC-12 member PC Maneet Bindra was exposed as an informant on her department, when she was seen handing a file to Hilton. Hilton later instructed her to obtain the password of DC Jamie Desford so she could download DI Matthew Cottan’s (aka The Caddy) dying declaration.
As Roz Huntley’s arm injuries worsened, she collapsed at home, and was later seen in hospital after having her arm amputated, as her wound had contracted MRSA.
Back at AC-12, the team were alerted when the remains of one of balaclava man’s victims were found, and they noted how the body had been kept in a deep freeze prior to disposal. Arnott quickly remembered how the body of Jackie Laverty (the woman who murdered after working for gangster Tommy Hunter in series one) was also kept in a freezer. Realising the cases might be connected, this allowed AC-12 to pursue a new line of enquiry and reopen their investigation.
Arnott then discovered that a blood sample in relation to one of balaclava man’s victims (which was planted by Huntley to frame Ifield as the serial killer) also contained white fibres from a forensic oversuit, and he realised that evidence had also been tampered with using the same blood. However, it lead him to wrongly conclude Nick killed Ifield with Huntley protecting him, and a warrant was issued for their arrest.
Realising the game could be up, and furious with her husband for allowing her arm to be amputated, Huntley managed to get one step ahead of AC-12 thanks to a heads-up from DC Jodie Taylor, and she arrested Nick for Ifield’s murder. After he’d been taken away, she ran one of her husband’s jumpers over a hairbrush she had taken from Ifield’s flat, thus setting evidence up against him.
During a visit to Hilton to explain the theory that Ifield was framed for the murders, Hilton played Hastings Cottan’s dying declaration that he had obtained from Maneet. It revealed Cottan had been working for a higher power - a corrupt officer within the force, known simply as ‘H’. Hilton accused Hastings of being ‘H’, and served him with a Regulation 15 Notice to formally answer the allegations.
Nick was later charged with Ifield’s murder after Huntley lead police to the contaminated jumper. Still not convinced he was guilty, AC-12 raced to find evidence against Huntley, and realised she would have left Ifield’s flat in a different set of clothes that would have been disposed of with the outfit she killed him in. They also deduced that the MRSA in Huntley’s now amputated arm would have been present in Ifield’s fingernails, with the bacteria therefore contaminating the balaclava he later placed over his head. They later tracked down all of the disposed items in an area Huntley had previously led a search on herself, and finally had enough evidence to arrest her for Ifield’s murder.
During questioning, she admitted to killing Ifield in self defence, but in a shock twist, she then arrested her solicitor - the aforementioned Lakewell - and exposed a web of lies.
She told of how Lakewell had spoken to Nick when Arnott came to his office, with Lakewell then tipping off Hilton. The ACC then ordered a man to attack Arnott. Huntley was able to prove this, as Hilton had been using a burner phone, which had the same number as the one he passed to her during their prior meeting.
As AC-12 rushed to find Hilton, it was revealed Desford too was a corrupt officer, tipping Hilton off that his crimes had been exposed.
During questioning, Lakewell revealed that there was not just one ‘balaclava man’, but many of them who are operating as part of an organised crime gang, exonerating not only Ifield of the murders, but original suspect Farmer too. This is the same gang who Cottan was working for as The Caddy, and is headed up by the mysterious ‘H’.
When AC-12 returned to their office having been unable to locate Hilton, it was being held up by a balaclava-clad gang, while Desford also held Arnott at gunpoint. The situation is quickly diffused.
The next day, Hilton is found dead, presumed as a suicide. With police concluding he must have been ‘H’, the investigation against Hastings was dropped. However, an odd camera angle and look from Hastings in the final moments of the series has led to the idea that he could really be ‘H’ after all...
11 November 2016 - Baswinder Kaur is murdered by Balaclava man
25 January 2017 - Leonie Collersdale is abducted by Balaclava man
9 March 2017 - Hana Reznikova is abducted by Balaclava man and rescued by the police that evening
17 March 2017 - Tim Ifield is murdered