"A counter-terrorist police raid kills an innocent man after mistakenly raiding the wrong flat. DS Steve Arnott refuses to take part in a cover-up. He is transferred to AC-12, an anti-corruption squad led by Superintendent Ted Hastings. Hastings informs Arnott that his first investigation will focus on DCI Tony Gates, the senior officer of TO-20, a unit of Central Police CID, who has just been made Officer of the Year. Hastings has received a tip-off that Gates has a suspiciously high clearance rate due to \"laddering\" \u2013 a process in which a number of different charges are placed on the same defendant to increase an officer's number of successful cases. Initially, Hastings interviews Gates on a minor charge \u2013 failure to report accepting a free breakfast for apprehending a mugger outside a local cafe."

"Gates continues his investigation into the double murder of the two suspected drug dealers, and interviews a less than co-operative witness \u2013 whom he forces to talk by threatening that, even if he does not talk, he will be labelled a police snitch. Dot and Deepak apologise for leaving the surveillance early. Meanwhile, Simon and Karen respond to a silent alarm at a block of flats and catch a burglar in the act. Leah interviews him, and during interrogation he also confesses to a number of other offences. Meanwhile, Arnott ponders why Gates has taken on the case of the hit and run. Whilst interviewing waitress Nadzia, she informs him that Gates' breakfast companion was tall, slim and dark-haired, and Arnott suspects she may have been describing Laverty. A search on the police database reveals a number of burglaries committed by the offender caught by Simon and Karen were on properties owned by Laverty Holdings PLC."

"Arnott speeds to Laverty's house following a tip-off from Fleming, only to find Gates supposedly searching the property, claiming that he has come to arrest Laverty. Whilst at the property, Gates tampers with evidence, hiding his whisky glass and wiping his prints from the bottle just as his team arrives. Arnott demands that Forensics dust the table for prints in the hope of proving that Gates was at the property before Laverty disappeared. Hastings and Arnott haul Gates in for questioning. Gates confirms that Laverty was an old flame, but he denies all knowledge of the money laundering scam. Gates flounders when Arnott asks him how he managed to park his car on Laverty's drive unless she let him onto the property, a hesitation which Arnott suspects is a sign of guilt. Hastings informs Gates' senior officer that he is to be taken off the Laverty case."

"Fleming suggests to Arnott that the link between Laverty's money laundering scam and Gates may be his children's school fees. Arnott visits the school, who later confirm that Laverty paid the deposit and the first full term in cash. Arnott agrees to return to AC-12. Meanwhile, Hastings decides to bring in the rest of Gates' squad to interrogate them over the 'laddering' charge. Morton refuses to comment, Fleming sticks to her cover and decides to back up Gates, but during questioning Hastings interrogates Cottan about the night of the surveillance on Greek Lane, and he claims that Gates ordered Deepak and him to end surveillance, unaware that this would create a prime slot in the timeline for the killers to strike. Meanwhile, forensic evidence at the crime scene and Arabic books belonging to the victims leads Gates to cook up a story regarding a possible terrorism plot, allowing him to wind up Arnott about the armed raid in which he failed to capture a terrorist cell."

"Gates returns to rescue Arnott from the hands of his torturers. Arnott, who is convinced that Gates is innocent of the murders and believes Gates can lead him to the killers, lies to Fleming and tells her that Gates did not play any part in his kidnapping. Meanwhile, with her cover blown, Fleming is forced to leave TO-20, but not before Morton expresses his disgust. Cottan is assigned as deputy SIO on the triple murder case, while Arnott's former counter-terrorism Chief Inspector is brought in to oversee the operation into apprehending the suspects. Fleming interviews Ryan, who denies any knowledge of 'Tommy' or the murder of Jackie Laverty. He does, however, reveal to Fleming that it was Gates who rescued Arnott. Fleming, upset that her colleague has lied to her, informs Hastings. Meanwhile, Arnott uses Morton to set up a midnight rendezvous with Gates. Gates tries to convince Arnott that he has never 'crossed the line', even once, and that he was forced to turn rogue and did not have any choice in the matter."

"A police convoy escorting a civilian under a witness protection scheme is attacked. The witness is hospitalised and all police officers are killed \u2013 with the exception of Inspector Lindsay Denton, who organised the operation at very short notice and who only informed DCC Dryden. Ted Hastings asks Steve and Kate to investigate Denton, but Kate initially asks not to take part. She explains that she trained together with Jayne Akers, one of the dead officers, but she has also been having an affair with Jayne's husband, Rich. She is replaced by Georgia Trotman, an ambitious but less-experienced officer who is troubled that the team are treating Denton as a suspect and delving into her finances. However, she and Steve quickly grow close, while Hastings tries to keep up appearances with his estranged wife. Denton, shunned by her fellow officers, is transferred to a missing persons unit, and Kate, undercover once again, becomes her assistant. Kate follows Denton to the nursing home where her mother lives and sees Denton ring the hospital where the injured witness is being treated. Georgia and Steve race to the hospital and confront a man disguised as a nurse. Steve is knocked unconscious; Georgia, who sees the killer's face, is thrown out of a window to her death, and the witness is killed."

"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. Denton begins her work in missing persons with the case of a vanished teenager named Carly Kirk. As they work together, Kate attempts to gain Denton's confidence, but Denton sees through this, assaults Kate and steals her phone. Denton is interviewed again by AC-12 and claims that she rang the hospital to ask that the witness exonerate her, but the team do not believe her and Hastings arrests her for conspiracy. At this point Denton turns the tables and gives evidence, on the record, that Steve made an unofficial romantic visit to Claire, Hastings has financial problems (something he previously cited as making Denton vulnerable to bribery) and Kate's mobile phone has an \"interesting\" call history (alluding to her affair with Richard Akers). Meanwhile, a story surfaces in the press that DCC Dryden took penalty points for his wife when she committed a driving offence."

"Denton is remanded in custody, where she is victimised by both staff and prisoners, having her hands badly burnt and warned not to speak to the authorities. Hastings requests witness protection records from Dryden, allowing AC-12 to discover the identity of the dead witness: Tommy Hunter, the gang leader previously arrested by Tony Gates. Cottan believes that the mole who betrayed the escort was in fact Jayne Akers, the liaison officer. Kate visits Denton, who knows about Kate's affair with Jayne's husband, which Cottan also suspects. Denton's accusations also cause friction between Hastings and Steve, and Dryden's driving offence continues to occupy the press. To draw attention away from himself, Dryden leaks the information that Denton is the officer under investigation. At the site where Denton stole Kate's phone and blew her cover, Kate discovers a body, apparently that of the missing Carly. She re-interviews Denton, who claims that Dryden, a married man, had an affair with her and has set her up. Kate discovers that Dryden and Denton did previously work together, lending some credence to this story. Cottan discovers from forensic accountants that Jayne Akers received a large sum of cash shortly before she died."

"Steve and Kate persuade Hastings to bring Dryden in for questioning but find the DCC uncooperative and frosty. They face a further problem when the Major Violent Crimes team bring in Richard Akers before they can do so, as Steve mistakenly assumed that Richard was unaware of his wife's corruption and so decided he was not a priority. When they do get to interview Akers, he reveals that Jayne kept recordings of Tommy as leverage and stored them at a PO Box. The recordings show Tommy threatening to inform on various parties, including corrupt police officers \"from the two-faced bastard right down to The Caddy\", unless they ensure his witness protection and immunity are maintained. A recording of Dryden publicly criticising the immunity Tommy and others like him receive suggest he is the corrupt officer in question, and AC-12 begin seeking the identity of 'The Caddy'."

"Denton manages to escape her kidnappers in violent circumstances that lead to the death of one and the hospitalisation of the other, and AC-12 find themselves forced to decide where their loyalties lie between Denton and Dryden. With the evidence mounting against the latter, the discovery of pictures showing Dryden and 15-year-old Carly Kirk in a compromising situation is the final nail in the coffin, resulting in Steve and Kate finally arresting the DCC. However, when presented with AC-12's evidence, Dryden still strongly maintains he is being set up. Meanwhile, Cottan approaches DC Morton from his old team. Morton is the officer who sold the story about Dryden's driving offence to the papers, and Cottan blackmails him into falsely saying Jeremy Cole, the deceased kidnapper, was nicknamed 'The Caddy' by fellow officers."

"Dryden claims that Denton was at the car park when he was photographed in the car with Carly. Steve gets closer to Denton but reveals to Kate that he is undercover, as she was earlier. Steve finds the pay-off money hidden among the possessions Denton took from her mother's room."

"Sgt. Danny Waldron and his team of armed officers are deployed to detain Ronan Murphy, a suspect wanted in connection with a plot to commit a gangland execution. Waldron orders a 'hard stop' on the suspect's vehicle, forcing the suspect to decamp into a nearby housing estate. Waldron gives chase, but catches up with him long before his team arrive. Waldron decides to shoot the suspect in cold blood, despite him having already surrendered his gun, and fakes the scene to make it look like he opened fire before he was shot. When Waldron's team arrive, he orders them to discharge their weapons. Under interrogation from AC-12, Waldron looks to have an answer for everything, until DS Arnott's line of questioning into his private life begins to spook him. Hastings agrees to send Fleming undercover as a new member of Waldron's squad in an attempt to uncover the truth."

"Kate Fleming attempts to provide first aid to the wounded Sgt. Waldron, who dies in the ambulance. The remaining members of the team, PC Rod Kennedy, PC Hari Bains and PC Jackie Brickford, are interviewed by AC-12 and claim Waldron shot himself despite their attempts to stop him. Hastings attempts to arrest them, but Gill Biggeloe convinces him to rescind the arrest. The police search Waldron's flat, finding Linus Murphy's dog (who Waldron took with him after the murder to help lead the police to Murphy's body) and DI Cottan discovers a box containing the stolen gun, a photograph of a young Waldron and other boys alongside Ronan and Linus Murphy, and the envelope, addressed to Arnott, containing Waldron's list. Cottan steals the list."

"Hastings authorises Fleming to begin surveillance of Hari Bains. She watches him meeting with Jackie Brickford; when Arnott interviews Bains' superior, Bains panics and is observed calling his contact from a payphone. Fleming asks Cottan to organise a second examination of Rod Kennedy's body, which Cottan intentionally neglects to do. The photo of Waldron and Ronan Murphy is traced to his time in Sands View boys' home. Arnott identifies Joseph Nash, another boy in the picture, and interviews him. Nash explains Linus Murphy was the caretaker at Sands View, that both men abused boys at the home, and that boys at the home were sexually abused by other men both at Sands View and at private parties. He identifies then-Councillor Dale Roach, but Roach is unable to stand trial, due to a stroke."

"Lindsay Denton is taken to AC-12, where she demands a formal apology. She reveals she has a recording of herself and DS Steve Arnott in a compromising situation that she will use against him as evidence of perjury if they do not comply with her requests. Denton is forced to work as a supermarket cleaner and later entraps an exploitative parole sponsor trying to pressure her into sex. Arnott \u2013 who is losing confidence within the department \u2013 eventually meets with Denton and asks for help in finding the link between Sands View and Tommy Hunter."

"Cottan continues to stack the deck against Arnott, first by pressurising PC Maneet Bhandra to hold off on key forensics (specifically the envelope found in Danny Waldron's flat), and then by spreading rumours surrounding Arnott's conduct, including telling Fleming he has a lead linking Arnott to Lindsay Denton's bribe money. Arnott is eventually suspended by a hesitant Hastings, to whom he reveals the redacted file that would have linked Ronan and Linus Murphy to Tommy Hunter. Hastings challenges Gill on this but is rebuffed. Meanwhile Fairbank is called in for interview and maintains his ignorance as he is presented with images of alleged abusers, such as councillor Dale Roach and real-life BBC DJ Jimmy Savile (who is not identified by name in the show). Joseph Nash is shown a photograph of Fairbank, ostensibly to identify him as an officer that had overlooked accusations, but his reaction suggests Fairbank was one of the abusers."

"Patrick Fairbank is arrested and interviewed on new information submitted by Joe Nash during an identity parade. During the interview, Hastings receives the list sent by Lindsay Denton which identifies Fairbank as one of the abusers. Meanwhile, Arnott awakens to find his pocketbook missing and his car stolen. As he reports the car missing, Hastings attends a call to a dockers' yard where the body of Lindsay Denton has been discovered in Arnott's car. Arnott is arrested on suspicion of murder. During questioning, evidence identifying Arnott as 'The Caddy', such as money from Lindsay's Denton's bribe and the mobile phone he used to contact Hari Bains, are shown to have been found in Arnott's gym bag. Arnott realises the evidence records relating to his hiring of a firearm have been doctored to suggest he did not return it. He knows somebody within the department has set him up, but has no way of proving it. Cottan then reveals, much to the surprise of Hastings and Fleming, he has suppressed the evidence found in the envelope left to Arnott by Danny Waldron, and that the evidence is a golf tee, supposedly confirming that Arnott is 'The Caddy'."

"DCI Roz Huntley is under intense pressure from her superiors to apprehend a serial murderer after months of fruitless investigation. When another young woman is abducted, Roz is on the scene to track down and charge a 24-year-old man. But doubts around the young man's guilt lead the chief forensic investigator to ask AC12 to investigate. Is Roz ignoring forensic evidence that might prove the young man's innocence?\n\nAC12's Supt Hastings, places DS Kate Fleming undercover inside Roz's team to dig deeper into the case. With DS Steve Arnott piling on pressure from the outside, Roz is forced to act decisively. A mother of two and wife to Nick, Roz will do anything to stop her life from unravelling."

"A dismembered body is discovered, and worries grow when DCI Roz Huntley fails to report for duty. AC-12 crank up their investigation into Operation Trapdoor, concerned that Michael Farmer will go to jail for crimes he didn't commit. With suspicions that Roz may have withheld crucial evidence, a new DCI is drafted in to run Operation Trapdoor.\n\nKate makes progress in her undercover role, but AC-12's investigation falters when they realise that their main informant, forensic investigator Tim Ifield, has also gone missing."

"As AC-12 wrestle with the new forensic evidence casting doubt over Tim Ifield, Kate is approached by another woman claiming to have been assaulted by Michael Farmer."

"Roz's husband Nick is brought in for questioning by AC-12. But while Nick denies any involvement in Steve's attack, his suspicions about Roz mount. Meanwhile, AC-12 discover new anomalies in the forensic evidence and issue DCI Roz Huntley with a second Reg 15 notice. AC-12's case appears watertight until Roz starts dismantling their evidence with inside information of her own."

"DCI Roz Huntley struggles to allay her husband's suspicions. AC-12 find a new angle to pursue their case against her."

"While Nick Huntley faces lengthy questioning, AC-12 remain convinced of Roz's involvement."

"Following the deadly hijack of a police convoy, AC-12 target a ruthless organised crime group known to have links with corrupt officers."

"Kate and Steve attempt to track down a lead. A fresh police leak enables another audacious raid, while Steve\u2019s 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."

"Tensions rise in the OCG following the events at Eastfield. With troubling questions hanging over the identity of \u2018H', suspicions mount at AC-12."

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

"In a nail-biting 90-minute finale, Hastings's secrets are laid bare. Will 'H' be revealed at last?"

"With time running out, AC-12 attempt to unmask 'H',the Fourth Man (or Woman) commanding the network of corrupt officers behind the murder of Gail Vella. But sinister and powerful forces appear intent on orchestrating a cover-up."