Line Of Duty recap: here’s what happened in series 1-4

Hit police corruption thriller Line Of Duty returns to BBC One on March 31, so get up to speed with our handy briefing on the story so far.

If you fancy watching it too, all four series are currently available on BBC iPlayer ahead of series five's arrival.

Warning: twists, turns and major spoilers ahead. Start the tape. Interview commencing…

Meet the AC-12 team

Who polices the police? This lot do. Line Of Duty follows controversial anti-corruption unit AC-12, operating within West Midlands Police Force and led by avuncular but relentless DSI Ted Hastings (Adrian Dunbar).

The series debuted on BBC Two in 2012 and opened with Hastings recruiting dogged DS Steve Arnott (Martin Compston), having been impressed when Arnott refused to collude with his anti-terrorism colleagues over the shooting of an innocent man. Undercover specialist DC Kate Fleming (Vicky McClure) completed the core team. Right, bring on the bent coppers.

The first chapter: big, sexy corruption

Series one saw AC-12 investigating the suspiciously high case clearance rate of DCI Tony Gates (Lennie James), leader of the self-styled “big sexy crime unit” and newly crowned Officer Of The Year.

While denying any wrong-doing, Gates helped out his mistress, businesswoman Jackie Laverty (Gina McKee), who claimed to have hit a dog while drink-driving. Except Gates soon realised Jackie actually killed a person during the hit-and-run: her own accountant, who’d discovered she was laundering money for mysterious gangster Tommy Hunter.

Before Gates could reluctantly arrest her, though, balaclava-clad intruders broke in and slit her throat, then planted Gates’ fingerprints on the murder weapon.

Hunter blackmailed Gates to keep quiet and do his bidding, or the incriminating knife would be handed to AC-12. Realising his career was over, Gates went rogue but eventually saw there was no way out. He asked his adversary Arnott to attest he was killed in the line of duty (show title klaxon!), so his wife would get death-in-service benefit and a police pension for life, then committed suicide by stepping in front of a lorry. Oof.

Enter The Caddy

In a head-spinning last-gasp twist, a member of Gates’s squad – smoothly slippery DS Matthew "Dot" Cottan (Craig Parkinson) – was revealed to be a gangland informant and inside man embedded in the police force. He leaked intel to aid criminal activity and had even murdered some drug dealers on Hunter’s behalf earlier in the series, before shifting suspicion onto Gates.

Codenamed “The Caddy” since a part-time job in his teens carrying Hunter’s golf bag, double agent Dot slyly advised Tommy to offer titbits of information to police, plead immunity and enter the witness protection programme. This wouldn’t be the last we’d see of the dastardly duo.

Steely Keeley steals second series

Series two in 2014 opened with an all-action armed attack on a police convoy escorting a protected witness. The witness was hospitalised and all officers killed, with the exception of DI Lindsay Denton (Keeley Hawes) – a bullied, desk-bound cop roped into the operation at short notice.

AC-12 investigated Denton’s role in the fiasco, with DC Georgia Trotman (Jessica Raine) joining the team. When she and Arnott visited the ailing witness in hospital, they stumbled across him being murdered by a man disguised as a nurse. The killer knocked Steve unconscious but with Georgia having seen his face, shockingly threw her out of the window to her death – meaning new star Raine lasted just one episode.

Manipulative Denton turned the tables on AC-12, having obtained information about Arnott’s habit of sleeping with witnesses, Fleming’s affair with a fellow officer and Hastings’s financial problems. Denton was later kidnapped by two corrupt cops, including the man who murdered Trotman, but escaped by killing one and hospitalising the other.

The dead witness from the convoy turned out to be Tommy Hunter, the gangster from series one. His killing was ordered by his criminal associates and organised by Cottan, out of fear that Hunter would inform on them.

Arnott discovered a holdall of cash at Denton’s house. She’d colluded with the ambush out of sympathy for a teenage girl being abused and pimped out by Hunter. However, Denton didn’t know they’d kill the officers too and, deliberately left alive to take the rap, was as much victim as villain. Still, the series closed with Denton being convicted of conspiracy to murder and given a life sentence. An acclaimed turn from Hawes with a gut-punch ending.

Caddy still out on the course

From recordings of Hunter, AC-12 learned of the existence of corrupt officer “The Caddy” and begin seeking his identity. However, Cottan blackmailed DC Nigel Morton (Neil Morrissey) from his former team into falsely stating that Denton’s deceased kidnapper was nicknamed “The Caddy” by colleagues due to his habit of wearing golf jumpers.

Having cast suspicion elsewhere, the elusive Cottan ended another series not only still at large but recruited full-time to AC-12 and promoted to DI. He had infiltrated the inner circle. Gulp.

Series three tackles Yewtree scandal

The third series in 2016 followed an even darker case than usual. It began with armed response team leader Sgt Danny Waldron (Daniel Mays) coldly shooting dead gangster Ronan Murphy in cold blood. Waldron then bullied his colleagues into covering it up and stonewalling AC-12’s inquiry.

Waldron later spied on Murphy’s funeral, followed his brother Linus home, then tortured and decapitated him in revenge for the Murphys being part of a ring who sexually abused him as a child. Returning to his flat, Waldron left a list of names including Ronan, Linus and Tommy Hunter (him again) in an envelope addressed to DS Steve Arnott.

When the squad returned to active duty, Waldron was shot in the neck and killed by one of his team during a drugs raid. Daniel Mays became another cast addition to last just one episode.

Searching Waldron’s flat, DI Cottan stole the list. Another member of the armed response team was killed, made to look like suicide, and framed for shooting Waldron. Meanwhile, Lindsay Denton appealed her conviction, was cleared and released.

Arnott enlisted Denton’s help in uncovering the paedophile ring, which included politicians, a senior policeman and even real-life DJ Jimmy Savile. Denton found a digital copy of Danny Waldron’s list of abusers but Cottan demanded at gunpoint that she hand it over. Denton refused and Cottan shot her in the head but was too late to prevent her emailing the names to AC-12. The paedophile ring was finally exposed and the surviving abusers were prosecuted.

Third time unlucky for The Caddy

Corrupt gangland fixer Cottan was the mastermind behind both deaths in the armed response team. He covered his tracks, but aroused Arnott and Fleming’s suspicions. Under mounting pressure, he manipulated evidence so Arnott was accused of being The Caddy and framed for Denton’s murder.

However, Fleming identified inconsistencies in his story and uncovered his blackmailing of Morton. Hastings put the squeeze on Cottan during interrogation, exposing his lack of an alibi for Denton’s murder.

In an adrenaline-pumping extraction scene, Cottan – at last unmasked as The Caddy – was broken out of AC-12 by corrupt officers and escaped, pursued by Fleming. After a frantic chase and a pang of conscience, Cottan took a bullet to save Kate's life. In a final act of redemption, he recorded the dying declaration that put the paedophile policeman behind bars.

Arnott had been suspended from duty but was cleared of all charges. Action heroine Fleming received an award for bravery and was deservedly promoted to Detective Sergeant. Too right.

Thandie gets handy

The fourth run in 2017, the first to be aired on BBC One, starred Hollywood actress Thandie Newton as ambitious DCI Roz Huntley, who was under pressure to get her career back on track after taking time out to raise a family. Her superiors – especially slippery ACC Derek Hilton (Paul Higgins), who first appeared in series one – were delighted when she cut corners to catch a masked serial killer known as “Balaclava Man”.

However, the suspect’s guilt and Huntley’s methods were soon called into question by forensics coordinator Tim Ifield (Jason Watkins). Huntley accidentally killed Ifield in self-defence (Watkins becoming another guest star not to survive episode one), in the process sustaining a nasty wrist injury which she tried to hide.

AC-12 investigated, with the newly promoted DS Fleming undercover on Huntley’s team. In retaliation, Huntley accused Hastings of sexism and questioned his Masonic connections, which were revealed last series.

Huntley’s husband Nick (Lee Ingleby) looked dodgy, as did his best mate, lawyer Jimmy Lakewell (Patrick Baladi). Suspecting this pair, Arnott tried to accost Nick at his office but was thrown down the stairs by Balaclava Man, leaving him in a wheelchair for the next few episodes.

Lakewell turned out to have framed one of his ex-clients as the serial killer to appease the balaclava-clad gang who have popped up throughout all AC-12’s investigations, right back to series one. They’d been blackmailing both corrupt Hilton and crooked Lakewell in their usual manner: with a frozen corpse covered in incriminating DNA.

Huntley’s worsening wrist was infected with MRSA and she collapsed, leading to her hand being amputated. Distraught upon regaining consciousness, she accused Nick of authorising the surgery in retaliation for their marriage troubles and her humiliatingly slapping him. She tried to get her own back by planting evidence on her husband’s clothes and arresting him for Ifield’s murder.

However, AC-12 untangled it all and nailed not only DCI Huntley (aided by traces of MRSA), but Lakewell and ACC Hilton too. Another Balaclava Man tried to bust out Lakewell at gunpoint but Hastings shot him. Huntley got 10 years for Ifield's manslaughter and the subsequent cover-up. Lakewell was jailed for perverting the course of justice but refused to testify. Hilton fled. For a day, at least.

Dropping the H bomb

In another tantalising twist, we discovered there’s a corrupt cop kingpin known only by the codename “H”, who heads a network of bent officers and was mentioned by Dot, aka The Caddy, during his dying declaration.

Hilton and Hastings both accused each other, but Hilton was found dead the next day – probably murdered by the balaclava gang for his silence, then staged to look like suicide. This left questions marks over Hastings, who remains in charge of AC-12 and was last seen looking shifty in his office. And that brings us right up to date. Phew.

And now for series five…

Early April 2019 brings the eagerly awaited fifth series, with guest star Stephen Graham playing another Balaclava Man, this one named John Corbett.

Corbett leads the armed hijacking of a transport vehicle in which three officers are murdered. Because the ambush required a police insider, AC-12 are called in to investigate. They find themselves facing an institutional cover-up but have a chance to nail the highest-level corrupt cops, including top dog “H”.

Hastings, Arnott and Fleming are all back. So are Line Of Duty’s trademark plot twists, shadowy conspiracies and tense interview scenes. See you on the sofa with bitten nails and a cushion to clutch.