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.

A police convoy escorting a civilian under a witness protection scheme is attacked, the witness hospitalized and all the police officers killed, except Inspector Lindsay Denton, who organized the operation at very short notice and who only informed DCC Dryden of the route. Ted Hastings asks Steve and Kate to interview her but Kate pulls out, claiming one of the dead officers, Jayne Akers, was a friend - though she has been having an affair with Jayne's husband Rich. Her place is taken by Georgia Trotman, who is troubled that the men are savage with their interrogation, treating Lindsay as a suspect and delving into her finances. Lindsay herself is moved to a missing persons unit but is shown to have a violent streak, attacking a noisy neighbour. Kate follows her to the retirement home where her mother lives and finds she has rung the hospital from a payphone. Georgia and Steve attend at the hospital where somebody is trying to kill the witness and who kills Georgia.

Following the deaths of Georgia and the witness it is Steve who gets a grilling as to the police presence at the hospital whilst Lindsay is accused of attacking her neighbour, which she denies. Steve 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, but whom she is unable to identify. Lindsay continues to search for vanished teenager Carly Kirk and Kate is drafted in as her assistant - in reality a move to try and get Lindsay to admit her guilt - but Lindsay sees through the ruse. After Kate's cover is blown Lindsay is questioned again and claims that she rang the hospital to ask that the witness exonerate her but she is disbelieved and Hastings arrests her for conspiracy. At her interview she points out that Steve visited Claire unofficially and Hastings has financial problems and a broken marriage that could make him just as culpable but she is nonetheless held in custody. Meanwhile DCC Dryden finds himself in the frame for a driving offence supposedly committed by his wife.

Lindsay is remanded in custody where she is victimized by both staff and prisoners. Meanwhile Steve and Kate discover the identity of the dead witness from blunt inspector Cottan. He was known as Tommy and Cottan believes that Jayne Akers, his liaison officer, was the real mole who betrayed the escort. Kate visits Lindsay, who knows about her affair with Jayne's husband whilst Cottan has similar suspicions. Lindsay's accusations also cause friction between Hastings and Steve, whilst Dryden's driving offence attracts the press so to save himself he strikes a deal naming Lindsay as the officer under investigation. Kate discovers a body, apparently that of the missing Carly, and returns to see Lindsay, who claims her ex-lover Dryden set her up. Kate makes a call which seems to support this whilst Cottan gets evidence to prove Jayne was on the take.

An arrogant Dryden is questioned regarding the traffic offence and is dismissive though Steve lets him know Lindsay has revealed that he knew about the convoy route and aims to pursue it further. Rich Akers is also pulled in after £20,000 of which he claims ignorance is found under the floor boards at his house. Hargreaves is convinced this was the bribe given Jayne to betray the escort and when Steve and Kate examine Jayne's car they discover a tracking device though Hargreaves is not keen for its revelation. Furthermore Cottan and Hargreaves see video evidence suggesting that Tommy had a grievance against Dryden, whose affair with Lindsay is now proved. Lindsay gets a day release from prison to visit her mother, in hospital after a stroke, but on the way back the prison van is hi-jacked and she is abducted by two policemen, Prasad, identified by Carly's parents as her apparent boyfriend, and Cole.

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?

When a criminal is shot dead by a police armed-response unit led by Sergeant Danny Waldron, AC-12 gather evidence suggesting it was a deliberate killing rather than self-defense.

Danny's armed-response unit come under increasing scrutiny as AC-12 feed Kate information to help her crack their story. Steve makes a shocking discovery, then his own past catches up with him.

DS Steve Arnott believes there is a dark secret in Danny Waldron's past. Then the spotlight falls on his questionable conduct in a previous inquiry - the Lindsay Denton case.

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

AC-12's loyalties are divided when DS Arnott comes under scrutiny from his colleagues.

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?

When DCI Roz Huntley captures a serial killer, Forensic Coordinator Tim Ifield alerts AC-12 to a possible miscarriage of justice.

A dismembered body is discovered, and worries grow when Roz fails to report for duty. After further questioning of Michael Farmer unearths crucial new information, AC-12 challenge the case against him.

As shocking new evidence is unearthed, AC-12 step up their investigation into the case against Michael Farmer.

The discovery of new forensic anomalies appears to provide AC-12 with a major breakthrough. Meanwhile, Nick Huntley comes under scrutiny.

AC-12's future hangs in the balance after Hilton bars them from investigating Roz Huntley. But Roz's problems are building closer to home.

AC-12 pit their wits against Roz Huntley in a climactic duel to find Tim Ifield's murderer and unmask Balaclava Man.

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 investigate DCI Jo Davidson's apparent mishandling of the murder of an investigative journalist.

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.

With time running out, AC-12 attempt to unmask 'H',the fourth individual commanding the network of corrupt officers.

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

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

AC-12 pit their wits against Roz Huntley in a climactic duel to find Tim Ifield's murderer and unmask Balaclava Man.

With time running out, AC-12 attempt to unmask 'H',the fourth individual commanding the network of corrupt officers.

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

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.

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

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.

When a criminal is shot dead by a police armed-response unit led by Sergeant Danny Waldron, AC-12 gather evidence suggesting it was a deliberate killing rather than self-defense.

As shocking new evidence is unearthed, AC-12 step up their investigation into the case against Michael Farmer.