Line of Duty: Who are the three leads and what has happened to them in past series?

The sixth season of Line of Duty is almost upon us, meaning it’s time to swot up on police acronyms and start unspooling your ball of string for the evidence wall.

Kelly Macdonald is the latest guest star to join the series, alongside regulars Vicky McClure, Martin Compston and Adrian Dunbar.

It’s been nearly two years since the hit police drama with the labyrinthine plot was on screens, so here’s a reminder of the three key team members in the show’s anti-corruption unit, AC-12, and what they’ve been up to over the past seasons…

Detective Sergeant Steve Arnott

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Known for: Speaking in inscrutable police acronyms and wearing unforgivable waistcoats.

Who plays him?

Martin Compston, 36, was born in Greenock, Scotland and is a former professional football player. His first ever film role was in Ken Loach’s Sweet Sixteen, and he has since appeared in series such as In Plain Sight, Traces, and The Nest.

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What’s happened to Arnott over the past five series?

In series one, Arnott led the investigation into DCI Tony Gates (the show’s first ever guest star, Lennie James). Arnott didn’t have an easy ride in this series: he was kidnapped and tortured, nearly losing his fingers. In the end, he and Gates developed an unlikely respect for each other, before the case came to a gruesome close with Gates walking in front of a truck.

Series two saw Arnott cheat on his girlfriend, DS Nicola Rogerson (Christina Chong), with a female witness. It was a digression that AC-12 target DI Lindsay Denton (Keeley Hawes) used against him when she was called in for interrogation. In a twist, and despite his penchant for a waistcoat, Arnott somehow managed to win over Denton and seduce her, too – but only to gather evidence against her.

Denton was freed on appeal in series three and, in another twist, she and Arnott joined forces to expose an historic child abuse scandal. Denton was killed, and DI Matthew “Dot” Cottan (Craig Parkinson) tried to frame Arnott as the corrupt copper – “The Caddy” – who’d murdered her. Arnott was suspended but luckily Fleming helped him out and he was exonerated.

In series four, Arnott was thrown down a flight of stairs while investigating DCI Roz Huntley (Thandie Newton). He was badly injured and temporarily wheelchair-bound.

Arnott was back on his feet in series five, but his back was still giving him pain and he started self-medicating with prescription pills. He won the trust of undercover cop DS John Corbett (Stephen Graham) and supported him in his mission to unmask the baddie, “H”. When it was discovered that police lawyer Gill Biggeloe (Polly Walker) was working for an organised crime group, Arnott shot the corrupt officer sent to silence her and received a commendation – but he still missed out on a promotion.

Detective Inspector Kate Fleming

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Known for: Saying “mate” and going undercover.

Who plays her?

Vicky McClure, 37, was born in Nottingham. She rose to fame as Lol in Shane Meadows’s This is England. She’s also appeared in Broadchurch, The Replacement, The Secret Agent and Alex Rider.

What’s happened to Fleming over the past five series?

In series one, Fleming went undercover to spy on Gates, but her cover was blown by DC Nigel Morton (Neil Morrissey), who punished Kate by hitting her with his walking stick.

There was more bad luck for Fleming in series two, who separated from her husband Mark and lost custody of their son Josh due to her long working hours. She went undercover again, but was caught out by Denton. She withdrew from that case and instead helped take down her corrupt boss, DCC Mike Dryden (Mark Bonnar).

Series three saw Fleming on yet another undercover mission. She joined the armed response team led by Sgt Danny Waldron (Daniel Mays), who was famously killed off after just one episode. She came to the rescue when Arnott was accused of being “The Caddy” and outed Cottan as the corrupt copper. There was a dramatic armed chase that ended in Fleming recording Cottan’s dying declaration. She received a commendation for bravery and was promoted from DC to DS.

In series four, Huntley was alerted to the fact that Fleming was undercover on her squad, so shut her out of their murder investigation. Fleming still managed to help nail down Huntley and her corrupt boss ACC Derek Hilton (Paul Higgins).

Fleming was promoted to DI for series five, meaning she usurped Arnott in rank. She made up with husband Mark but long working hours created tensions between them again and they split once more. Fleming was forced to report her beloved boss Hastings when he went rogue on an unauthorised undercover operation.

Superintendent Ted Hastings

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Known for: His catchphrases, such as “Mother of God!”, “Give me strength”, and “We’re interested in one thing and one thing only and that’s catching bent coppers”.

Who plays him?

Adrian Dunbar, 62, was born in County Fermanagh, Northern Ireland. He has received a Bafta nomination for Line of Duty and also starred in Cracker, The Hollow Crown, Ashes to Ashes, Broken, and Blood.

What’s happened to Hastings over the past five series?

In series one, AC-12 boss Hastings sent in Arnott and Fleming to nail down Gates.

The second series saw Denton embarrass Hastings by exposing his huge debt problems, suggesting he was susceptible to bribery. His financial woes also caused tensions in his marriage to childhood sweetheart Roisin (Andrea Irvine). Hastings recruited Cottan to AC-12, unaware that he was “The Caddy”.

Series three saw Hastings’s wife Roisin leave him after he lost their life savings. He was still in love with her, so turned down the advances of police lawyer Biggeloe. He was spotted sharing a Masonic handshake with a retired vice cop implicated in the historic paedophile ring AC-12 were investigating, making Arnott suspicious about whether his boss was involved, but he seemingly wasn’t as the culprits were taken down anyway.

In series four, Huntley accused Hastings of sexism and of being a Freemason while she was being interrogated. Hilton put Hastings under investigation for being “H”, an officer suspected of instructing corrupt coppers. But then Hilton himself was exposed as corrupt and found dead, and it was widely assumed that he was in fact H.

Hastings began to look dodgier and dodgier in series five. He was handed an envelope bursting with cash, apparently as compensation for the failed property investment that had got him into money trouble. He also appeared to be in communication with Corbett’s gang via his laptop, which he later had destroyed. After Corbett – whose mother was somehow linked to Hastings during The Troubles – attacked his estranged wife Roisin, Hastings ordered Arnott to shoot him but Arnott refused. Biggeloe framed him for being corrupt and he was suspended from duty. He was then reinstated at AC-12 after a final written warning.

Line of Duty returns on Sunday 21 March on BBC One at 9pm.