Having been duped into covering up Jackie's hit and run, Gates is desperate to sever ties with her. But as Arnott and Fleming close in on his secret, Gates is plunged into an even more dangerous situation.

Gates, knocked unconscious by the attackers, tampers with evidence, hiding a whiskey glass before police arrive to find Jackie has vanished. He claims he has just got there himself. Hastings and Arnott quiz him on his relationship with Jackie and he admits she was an old flame but denies knowledge of her money laundering. Arnott goes over the top in confronting him with evidence to suggest he was at Jackie's house when she was killed but makes himself look desperate. Gates is taken off the case and later, whilst investigating Wes's murder, is grabbed by masked men and an evil child who threaten to frame him for Jackie's murder unless he takes over her money laundering racket.

Whilst the money laundering investigation is passed to Fraud Squad, Gates is still hounded by his blackmailers. Kate wants to connect him to Jackie's murder and sets out to gain his trust to find further proof though Gates' loyal colleague Morton rumbles her. Hastings and Arnott interview Gates' team and learn from D.S. Cottan that he diverted them from attending the drug dealers' murders, leading them to believe that Gates knew that the dead pair were working with Jackie though Gates persuades his gullible superior that the couple were in fact terrorists building a bomb. Arnott is lured to a warehouse by a telephone call purporting to lead to more information but finds himself captured by the gangsters who, as Gates walks away, tie him up and torture him for his knowledge.

Gates returns to rescue Arnott but leaves before Kate, her cover now blown, arrives to find evidence that a body had been stored at the warehouse. Arnott recalls that the chief torturer was called Tommy and admits to Kate that Gates saved him. She believes he is weakening in his attitude to Gates, whom he goes to see in secret, confirming his view that Gates is innocent of all the murders. Convinced that Gates is in danger but can bring the killers to justice Arnott persuades Hastings to let Gates set up a meeting with Tommy, recorded on a wire. Arnott and Kate intend to meet up with them but before they can do so Gates abducts Tommy to force a confession before committing an extreme act in the line of duty.

When three police officers are shot dead during the hijack of a seized drugs transport, AC-12 move in to investigate possible police collusion.

A fresh police leak enables another audacious raid, while Arnott's attempts to gather information put him in a dangerous situation.

Corbett's most daring plan yet requires McQueen to exploit links with corrupt police officers. Meanwhile, the AC-12 team make a shocking discovery.

Tensions rise in the OCG following the events at Eastfield. With troubling questions hanging over the identity of the mysterious H, suspicions mount among the AC-12 team.

As the hunt for H intensifies, Steve and Kate wonder if Hastings is guarding a dark secret.

The search for the mysterious H reaches its climax as long-buried secrets finally come to light.

AC-12 suspect a cover-up as an officer on DCI Davidson's team appears to have sabotaged the inquiry into Gail Vella's murder.

A new witness comes forward claiming to have crucial evidence in Gail Vella's murder. Kate is torn between her loyalty to Jo and to AC-12.

DCI Davidson faces increasing pressure from all sides. AC-12's attempt to interview a key witness goes horribly wrong.

AC-12 link Gail Vella's murder with a historic case of police corruption, and Kate hatches a plan to tell once and for all if Jo is bent.

As AC-12 struggle to deal with the repercussions of tragic events, Hastings makes one final bid to uncover institutionalized corruption.