After a bungled raid which kills an innocent man young DS Steve Arnott faces disgrace. However he is transferred to the anti-corruption squad, whose head, superintendent Ted Hastings, gives him a "very special case" to investigate - DCI Tony Gates, recently awarded a medal for heroic conduct but with a suspiciously high arrest rate. Hastings tries to get Gates on minor bribery charges, in vain, and is unaware that the married Gates has covered up a fatal hit-and-run by his lover, businesswoman Jackie Laverty. Meanwhile DC Kate Fleming joins Gates's team and tells Steve she is investigating him from the inside following the deaths of two drug dealers.

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.

Lindsay wakes up in a lock-up and is questioned by her abductors. Prasad soon kills his accomplice, Cole, but Lindsay outwits him and makes him admit on the phone that he and Cole were working for Dryden, who set up the ambush. Steve locates her at her dead mother's bedside and she is granted bail, unaware that Prasad, whom she believed that she killed in self-defense, is still alive and demanding immunity. Cottan brings in his old mate Morton for some dirt on Dryden and it is revealed that Dryden had sex with Carly Kirk. He is arrested and questioned, admitting that he had a brief affair with Lindsay, who became vindictive when he ended it. Steve also finds out that Lindsay has had an abortion.

Prasad claims that Dryden wanted Tommy silenced as he was blackmailing him about Carly, hence the ambush. However the order was to spare Lindsay. Hastings charges Dryden for the driving scam but has insufficient evidence for the weightier charges as Dryden apparently proves that the last he saw of Carly was when he put her on a train. Kate is surprised that Steve seems to believe in Lindsay's innocence until he admits that he is purposely trying to gain her confidence to trap her. When she is shown to have accepted a proposal by a colleague to snare a child abuser she seems less innocent than she claims but is she really guilty of conspiracy to murder?

Fresh testimony launches AC-12 back on the trail of the Caddy, a corrupt officer with links to organized crime.

Steve is arrested on suspicion of murder but continues to protest his innocence. Kate's undercover operation unveils unexpected facts, but will it hold the key to uncovering the identity of the Caddy?

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.

Fresh testimony launches AC-12 back on the trail of the Caddy, a corrupt officer with links to organized crime.

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.

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.

Lindsay wakes up in a lock-up and is questioned by her abductors. Prasad soon kills his accomplice, Cole, but Lindsay outwits him and makes him admit on the phone that he and Cole were working for Dryden, who set up the ambush. Steve locates her at her dead mother's bedside and she is granted bail, unaware that Prasad, whom she believed that she killed in self-defense, is still alive and demanding immunity. Cottan brings in his old mate Morton for some dirt on Dryden and it is revealed that Dryden had sex with Carly Kirk. He is arrested and questioned, admitting that he had a brief affair with Lindsay, who became vindictive when he ended it. Steve also finds out that Lindsay has had an abortion.

Prasad claims that Dryden wanted Tommy silenced as he was blackmailing him about Carly, hence the ambush. However the order was to spare Lindsay. Hastings charges Dryden for the driving scam but has insufficient evidence for the weightier charges as Dryden apparently proves that the last he saw of Carly was when he put her on a train. Kate is surprised that Steve seems to believe in Lindsay's innocence until he admits that he is purposely trying to gain her confidence to trap her. When she is shown to have accepted a proposal by a colleague to snare a child abuser she seems less innocent than she claims but is she really guilty of conspiracy to murder?

Steve is arrested on suspicion of murder but continues to protest his innocence. Kate's undercover operation unveils unexpected facts, but will it hold the key to uncovering the identity of the Caddy?