LINE of Duty is back on the beat for its sixth series.
The BBC crime drama stars Vicky McClure, Martin Compston and Adrian Dunbar as AC-12 investigators.
🔵 Read our Line of Duty live blog for the latest updates
What happened in Line of Duty season 1?
The first series debuted in 2012 sees an innocent man getting killed after a counter-terrorist police unit raids the wrong flat.
After refusing to participate in a cover-up, DS Steve Arnott (Martin Compston) is recruited by Superintendent Ted Hastings (Adrian Dunbar), head of anti-corruption unit AC-12.
His first assignment is to investigate DCI Tony Gates (Lennie James), a high-flying officer with a suspiciously impressive success rate.
With the help of undercover officer Kate Fleming (Vicky McClure), Arnott discovers Gates has indeed been covering up for a deadly hit and run.
Realising his career is over, Gates takes his own life - but the corruption does not die with him.
Another officer in his team, DS Matthew "Dot" Cottan (Craig Parkinson), is revealed to be an inside man for gangster John "Tommy" Hunter (Brian McCardle).
They gave Dot the codename of 'The Caddy' because he used to carry the mobsters' golf kit as a youngster.
However Dot is better than Gates at covering his tracks.
He told Tommy what to say to the police, leading him into witness protection.
What happened in Line of Duty season 2?
DI Lindsay Denton - as played by Bodyguard actress Keeley Hawes - led the drama in this season.
The character was involved in a police convoy ambush which lead to the death of some colleagues as well as a member of public who was under witness protection.
But the anti-corruption unit become suspicious of DI Denton as it transpired that she was battling a crippling debt problem, and clock that she would have realised as the sole survivor she may be entitled to a big pay-off.
Arnott and Fleming from the previous series continue their hard work and focus their attentions on the shifty DI.
Fleming takes on the case after ending her affair with one of the deceased colleague's husband's.
While Arnott also embarks in a steamy relationship with sexy new colleague Georgia - until she meets a tragic fate.
And - perhaps unsurprisingly - the series ends with Lindsay Denton being found guilty as charged.
But it does come with a twist, her motive wasn't so much financial as it was covering up a bad decision that lead to the death of four colleagues.
What happened in Line of Duty season 3?
This series saw officer Daniel Waldron investigated by AC-12 after a suspect was shot and killed.
But the first episode ended with the officer himself getting killed.
The unit uncovered a paedophile ring at Daniel's old school involving some high-powered figures including Jimmy Saville.
It turned out that Waldron had been taking revenge on his previous abusers at the boys' school.
But all the while, Dot was trying to stifle the case and stop it from coming out.
Series 2's DI Lindsay Denton (Keeley Hawes) also made a return after not being found guilty for conspiracy to murder.
And The Caddy became the subject of AC-12's interest after two series of dodging justice.
Unfortunately, he met his fate in a high-speed car chase as he tried to escape police officers.
What happened in Line of Duty season 4?
AC-12 turned their attentions to Roz Huntley (played by Thandie Newton) in this series.
They explored a potential miscarriage of justice when the pressure to close a case resulted in the wrong man - Michael Farmer (played by Scott Reid) - getting locked up.
Is mother-of-two Roz responsible for an innocent man rotting behind bars?
It also saw the exoneration of Ted, Michael Farmer and Hana Reznikova who were unfairly implicated in the conspiracy.
Fleming, Arnott and Hastings unmasked Assistant Chief Constable Derek Hilton for his pivotal role in the series' conspiracy.
They were convinced Hilton was the mystery 'H' who was complying with powerful criminals, but the finale ended with Hilton dying.
What happened in Line of Duty season 5?
An Organised Crime Group (OCG) hijacked a police convoy transporting seized drugs and killed three armed officers; AC-12 suspect that undercover police may be involved.
They discover that the undercover officer is DS John Corbett and not, as originally assumed, Lisa McQueen.
AC-12 discover that Corbett has links to Northern Ireland.
The raid was masterminded by Corbett, who was posing as a criminal in order to identify the mysterious H.
Before Corbett can unmask H, he is betrayed by second-in-command Lisa McQueen (Rochenda Sandall) and brutally murdered.
The ensuing fall-out sees Ted Hastings suspended, investigated and charged with conspiracy by DCS Patricia Carmichael (Anna Maxwell Martin).
He was investigated about the £50,000 in banknotes found in his hotel room.
But, during a tense interrogation, Hastings turns the tables by exposing lawyer Gill Biggeloe (Polly Walker) as part of the OCG.
The series ends with Hastings back in charge at AC-12, after receiving a final written warning.
McQueen was given immunity.
Unnervingly Ryan Pilkington, Corbett's killer, has been inducted as a student police officer (you'll remember him as a kid from season 1).
Are the plots based on true stories?
Some storylines are a case of art imitating life.
Speaking about these plot inspirations, creator Jed Mercurio told the “Well, there are actually, really relevant examples in British legal history of Stefan Kiszko and you should look up who his solicitor was.
"It’s interesting - and the other one is Barry George, who convicted and then acquitted of the Jill Dando murder. So, there are actually, really relevant correlates in the British system.”
MORE LINE OF DUTY
When is Line of Duty series six on BBC One?
Line Of Duty ends tonight, Sunday May 2, 2021.
You can catch it on BBC One at 9pm.
After it has aired it will be available on iPlayer.
The first episode of season six aired last Sunday, March 21.