List of ''Line of Duty'' episodes

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" – 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 – 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 – 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.

Arnott persuades Hastings to let him use Gates to get to Tommy. Hastings agrees to put a tap on the phone network used by Tommy until one of his many phone numbers is re-activated. Arnott primes Gates, who convinces his wife to let him spend one final hour with his children. When the phone number is re-activated, Gates, Arnott and Fleming trail Tommy to the Edge Park golf club. Gates arrests Tommy. Under interrogation, Tommy reveals that his boys were responsible for the murder of Jackie Laverty and also took out the three drug dealers. Tommy claims that the murders were committed by 'an inside man, someone special to him' who 'always follows orders' and who would admit to them if he forced him to. As Arnott and Fleming catch up with them, Gates realises that his career is over and decides to commit suicide, telling Arnott to inform his wife that he was killed in the line of duty. After Tommy is arrested, Cottan has a quiet word with him. Tommy says, 'You tell me how I'm going to play this one', to which Cottan replies, 'Offer them all the information you can and plead immunity', seemingly confirming that Cottan is Tommy's inside man, later revealed as 'The Caddy', having been recruited as a teenager when he caddied for Tommy at a local golf club.

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.

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'.

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.

In flashback, the events prior to the ambush:
Denton follows Dryden to the reception, where Carly meets and attempts to seduce him. Denton has a brief conversation with her in the ladies' cloakroom and then follows Carly and Dryden to the car park, where she sees Carly fellate Dryden and that she is working for Tommy. Some days later, Denton approaches Tommy but is surprised to find him accompanied by DS Jayne Akers. Akers visits Denton at her home to request help in handing Tommy over to his criminal associates, who want to kill him out of fear he will inform on them in witness protection. With a pay-off, and appealing to Denton's desire to protect Carly from her pimp and abuser, Akers persuades her to assist in the handover. In reality the handover is being planned by Cottan, the mastermind who is known only to Akers and Tommy. He orders two corrupt officers to ambush the convoy, who follow it using a tracker placed on Denton's vehicle. Akers and Tommy are both killed to protect Cottan's identity, Denton is left alive to take the blame.

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.

Waldron's team are tense as a result of being bullied into collaborating with Waldron, and Fleming identifies PC Harinderpal 'Hari' Bains as a possible informer, but Bains refuses to testify. After pressure from AC-12 department lawyer Gill Biggeloe, Waldron's team are returned to active duty along with Fleming. On their first call, they are tasked with raiding a house where drugs are kept. Waldron steals an illegal firearm, then stops Fleming from aiming her gun at a child who surprises them. Waldron demands Fleming transfer out of his squad.

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.

Arnott agrees to take over the investigation of Waldron while Cottan focuses on his colleagues. Arnott locates Linus's home and finds his tortured and decapitated body, then finds the head in a storage room rented by Waldron. Waldron's team-mates find themselves suspected by their colleagues, and Fleming pressures them further by suggesting she knows they murdered Waldron, and indicating she will tell AC-12 about a radio call Bains made to Waldron moments before his death, which was not included in the reports. Meanwhile, Lindsay Denton has appealed against her conviction for conspiracy to murder Tommy Hunter, and Arnott is called as a witness. He denies having a sexual relationship with Denton, but the suggestion puts strains on his relationship with his girlfriend, DS Sam Railstone. Scared by pressure from Fleming, Kennedy demands to meet Bains in a disused warehouse, blaming Bains for their situation. This results in a brief physical confrontation. Denton claims in court that she had sex with Arnott.

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.

Brickford visits Fleming to find out what AC-12 has been asking her. Fleming reveals Bains has been making phone calls, seemingly to Brickford. Brickford confronts Bains, who cannot explain the calls. Brickford returns to AC-12 and confesses Waldron was shot in a struggle that Brickford and Kennedy were involved with. Only Bains saw how it began. Bains claimed Waldron had aimed at him, and that Bains grabbed the gun in self-defence. Brickford also confesses to lying about Ronan Murphy's death. Hastings has Brickford's police contract terminated and warns her she may yet be charged. Meanwhile, Arnott orders a second post mortem on Rod Kennedy's body. It indicates he was murdered. AC-12 moves to arrest Bains and the firearms unit are deployed to arrest him, but his contact (revealed to be Cottan) warns him and he escapes. He travels to the warehouse where Kennedy was hanged, and finds Cottan (unaware that Cottan is his contact). As the firearms team approach, Cottan handcuffs and injures himself to fake a fight. Bains is arrested and the police find a noose, supporting Cottan's story that Bains was attempting to hang him.

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 – who is losing confidence within the department – eventually meets with Denton and asks for help in finding the link between Sands View and Tommy Hunter.

DI Matthew Cottan is awarded with a commendation for his perceived bravery arresting a corrupt police officer, Hari Bains. He then proceeds to tell Gill and DC Kate Fleming he is concerned about Steve's behaviour and that Steve is carrying a firearm in his jacket. Bains confesses to killing Danny Waldron on the demands of his unknown contact, but insists he did not murder Rod Kennedy. Steve and Kate confront Ted, saying they believe they should question a retired officer on the subject of allegedly reporting to a mother that a sexual-abuse complaint of Dale Roach was dealt with, resulting in Roach being charged and sent to prison. The mother saw Dale Roach in the newspaper some time later, so knew she had been lied to. The death of a former social worker at Sands View, ruled as a suicide, is reinvestigated due to allegations that he attempted to report the abuse to police and may have been murdered to prevent his case being pursued. They eventually interview former vice chief superintendent Fairbank, who claims to remember no details of the case. He and Hastings share a handshake that Arnott identifies as Masonic.

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.

Arnott and Lindsay Denton investigate the possibility that Danny Waldron hid another copy of his list of abusers. Denton realises the list is likely kept digitally, so she tricks Arnott into pursuing a false lead while she, having stolen Arnott's pocketbook, goes to an internet cafe to access Waldron's emails. She finds a photograph of the list in his drafts folder and transfers it to hers as insurance.

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'.

Fleming informs Hastings she has been carrying out an undercover operation into the officers of AC-12, under the orders of a superintendent from AC-3. She highlights inconsistencies within Cottan's line of questioning against Arnott. Having spoken with DS Nicola Rogerson, she identifies Cottan was the one who fed AC-12 the doctored file on Ronan Murphy. Hastings confronts Gill Biggeloe, who confirms it was Cottan who gave her the file. Maneet later reveals to Fleming it was Cottan who told her not to inform Arnott of the forensic findings of the envelope found in Waldron's flat. Fleming also uncovers CCTV which confirms Arnott's car was being driven on false plates shortly before the time of the murder.

Cottan is brought in for questioning. He denies all knowledge of the doctored file and claims Arnott must have been the one responsible, having previously had a relationship with DS Rogerson. Hastings pressures Cottan on the suppression of the golf tee from evidence, as well as the fact that no blood was found on the tee despite blood being found on both the envelope and the list. Cottan claims he only removed it to prevent Arnott from becoming aware he was investigating him. Fleming reveals new forensic evidence against Cottan – fibres from the rope found at Rod Kennedy's murder scene, in the boot of his car. Cottan strenuously denies any involvement with Kennedy's murder. Hastings probes him on evidence submitted by Nigel Morton, which suggests he pressured Morton into wrongly identifying 'The Caddy', and also used a secret mobile phone. Phone records reveal he previously called Tommy Hunter several times in the months leading up to his death. Fleming presses Cottan on the lead he claimed to have on Arnott linking him to Lindsay Denton's bribe, which Hastings points out could not have been the money in Arnott's gym bag as that was not discovered until after Denton had been killed, the morning after Cottan alleged to have evidence against Arnott.