Line of Duty series 1-5 recap: What has happened so far in the BBC drama?

To help you out, we've provided a recap of all the major events from each of the previous series below – read on for everything you need to know about what's happened so far in the BBC drama.

What has happened in Line of Duty so far?

Season One

The first series introduced us to the three characters who have continued to be at the centre of the drama ever since: Superintendent Ted Hastings (Adrian Dunbar), the commanding officer of Anti Corruption Unit 12 (AC-12), Steve Arnott (Martin Compston), a recent transfer from the Counter-Terrorism Unit, and Kate Fleming, an AC-12 officer who specialises in undercover work.

The task for AC-12 in the first run was to investigate the recently-named Officer of the Year Tony Gates (Lennie James) – who had raised suspicions due to his extremely high arrest rate.

It transpired that Gates had agreed to help his lover Jackie out of a spot of bother after she'd 'hit a dog' while drink-driving, but what she hadn't told him is that what she had actually deliberately run over her own accountant after he'd discovered she was laundering money for local gangster Tommy Hunter.

Naturally, things didn't end well for Gates – who was blackmailed and framed by Hunter after the latter had murdered Jackie. Towards the end of the series, Gates took his own life after leading AC-12 to Hunter and successfully securing his arrest.

There was still time for one more revelation though – at the very end, it is revealed to the audience that DS Matthew “Dot” Cottan (Craig Parkinson), one of Gates’s subordinates, was the corrupt officer codenamed 'The Caddy' and had been secretly working for Hunter – eventually helping him into a witness protection scheme.

As the season ends, AC-12 remain unaware of Dot's treachery...

Season Two

Season two saw the introduction of perhaps Line of Duty's most memorable guest star to date – Keeley Hawes as DI Lindsay Denton, the sole survivor of an armed attack on a police convoy that had been transporting a protected witness.

Naturally, AC-12 suspected that Denton had been acting as an insider, and she became the subject of an intense investigation – albeit one in which she initially frequently got the upper hand.

It eventually emerged that Tommy Hunter had been the protected witness in question – and that it was Dot who had engineered the attack on the convoy with the intention of murdering Hunter to protect himself, lest his cover was blown.

Denton wasn't entirely innocent – she'd acted as an accomplice after being convinced of Hunter's evil, but hadn't known that she would be framed for the attack.

As the series ended, she was convicted of conspiracy to murder, while Dot – who had already been helping AC-12 with their investigation – was invited to join the team permanently, ironically charged with finding the identity of the bent copper known as "The Caddy"...

Season Three

The big guest star at the start of series three was Daniel Mays, playing armed response team leader Sergeant Danny Waldron – but in a huge twist, he didn't even make it to the end of the first episode.

Waldron had a list of names of people in various positions of power who had sexually abused him and his schoolmates as a child and was planning on taking revenge, but he was killed by one of his own teammates in the process.

Before his death, he'd tried to hand AC-12 a copy of his list, but it was destroyed by Dot before Arnott could get his hands on it.

Then came the surprise return of Lindey Denton, whose conviction was overturned on appeal, leading her to assist Arnott in getting to the bottom of the child abuse case.

In doing so, she made the discovery that Dot had been the corrupt insider all along, leading to a showdown between the pair which tragically ended with her being shot dead – although thankfully she had been able to deliver a digital copy of Waldron’s list of abusers to AC-12 beforehand.

Meanwhile, Arnott had been framed as the insider by Dot, but Cottan's plan backfired and his real identity was revealed in an epic interview – leading to a dramatic chase as he tried to make his escape.

At the end of that chase, Dot eventually died in surprisingly heroic circumstances by jumping in front of bullets from a mystery assailant to save Fleming’s life, recording his ‘Dying Declaration’ for his colleague before passing away.

Season Four

Season four introduced another big-name guest star to the mix – this time in the shape of Thandie Newton as Roz Huntley, a detective who was tracking a serial killer.

Her conduct had raised alarm bells for forensic specialist Tim Ifield (Jason Watkins), who informed AC-12 that he believed something was amiss in her investigation – and not much later he turns up dead, not to mention missing three fingers.

Later, Huntley admits that she was responsible for Ifield's death after she accidentally killed him during a fight, and is sentenced to jail.

Meanwhile, Arnott is severely injured after being pushed down a staircase by a mystery assailant – leaving him in a wheelchair, while we finally learn the content of Dot's dying declaration from the end of season three.

It turned out that he had told Kate that there was a corrupt police officer, codenamed 'H', who was embedded deep within the force and was the main mastermind behind a huge conspiracy.

Naturally, anyone with a surname beginning with H became a suspect, including Ted Hastings himself, as well as ACC Derek Hilton – although the latter was killed off at the end of the series in an apparent suicide.

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Season Five

This brings us to the most recent series – for which Stephen Graham took on the main guest role, playing undercover cop DS John Corbett, who appeared to have gone rogue while infiltrating an organised crime group (OCG).

Corbett had been assigned to find the identity of 'H' while working undercover, but he eventually became the latest guest star not to make it through a whole series – killed after being double-crossed by Lisa McQueen (Rochenda Sandall), a member of the OCG.

Meanwhile, Hastings had emerged as a top suspect in the 'H' investigation – in no small part due to complicated links between himself and Corbett in the past – and was grilled by DCS Patricia Carmichael (Anna Maxwell Martin) after being suspended from duty.

Eventually, Hastings appeared to be exonerated after he implicated lawyer Gill Biggeloe, who it transpired had been attempting to frame him – although it's fair to say there were still a few question marks hanging over him by the end of the series.

One of the biggest revelations at the end of series five concerned new information about Dot's dying declaration – it turned out that he was communicating (using Morse code) that there were actually not one but four high-ranking police staff working with the OCG.

We now know that three of them were Dot himself, Gill Biggeloe and Derek Hilton, but as for the fourth? It's still anyone's guess.

What's more, at the very end of the series we saw Corbett’s murderer Ryan Pilkington enrolling as a student police officer – suggesting that AC-12 will still have plenty of bent coppers to deal with for a while yet...

Line of Duty begins on Sunday 21st March at 9pm on BBC One. Take a look at the rest of our Drama coverage, or check out what else is on with our TV guide.