Line of Duty, the BBC’s hit drama about police corruption, returns to our screens on Sunday.
The fourth series introduces Thandie Newton as DCI Roz Huntley, the latest officer to come under investigation by anti-corruption unit AC-12.
Written by Jed Mercurio, the series has earned a devoted following thanks to its shocking plot twists, edge-of-the-seat moments and starry ensemble.
But what of viewers coming to the drama cold? Here’s a handy bluffer’s guide to help them get up to speed.
Warning: this article contains plot spoilers. Do not read on if you do not want to find out what occurred in previous series of Line of Duty.
SERIES ONE (2012)
After refusing to participate in a cover-up, DS Steve Arnott (Martin Compston) is recruited by Superintendent Ted Hastings (Adrian Dunbar), head of 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 Tony has indeed been covering up for a deadly hit and run.
Realising his career is over, Tony 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).
Yet Dot – also known as “The Caddy” – is better than Tony at covering his tracks…
Shocking moments: Tony’s girlfriend Jackie (Gina McKee) having her throat cut; Arnott being tortured; Gates killing himself.
Memorable dialogue: “You take a shot at the king, make sure you kill him, son!” (Tony Gates to Steve Arnott)
SERIES TWO (2014)
A police convoy is ambushed by two gunmen who leave three officers dead and their prisoner hospitalised.
The prisoner, who is later killed along with another officer, is revealed to be Tommy Hunter, who had been under witness protection.
DI Lindsay Denton (Keeley Hawes), who had been in charge of the convoy, comes under suspicion and narrowly survives a murder attempt.
It is later revealed that she’s a pawn in a plan cooked up by Cottan and his criminal associates, who wanted Tommy silenced in order to protect Dot’s secret identity.
The series ends with Denton in prison and Cottan installed at AC-12, safely hiding in plain sight…
Shocking moments: DC Georgia Trotman (Jessica Raine) being thrown out of a window; Denton attacking a neighbour with a bottle.
Memorable dialogue: “People have underestimated me my whole life!” (Lindsay Denton)
SERIES THREE (2016)
A police raid ends with Sergeant Danny Waldron (Daniel Mays) gunning down a suspect in cold blood, then making it look as if the dead man shot first.
Danny is then himself killed, but not before alerting Fleming to the existence of a list of people involved in a child abuse ring.
Finding the list and seeing Tommy Hunter’s name on it, Cottan destroys it while making it look as if Steve is the mysterious “Caddy”.
But he is finally exposed by DI Denton who, having been acquitted of conspiracy to murder, dies at his hand emailing the list to AC-12.
After a lengthy interrogation, Cottan escapes, only to die, semi-heroically, stopping Fleming being shot by one of his associates.
The series ends with Steve returning to duty, the chief abuser being convicted and Kate receiving both a commendation and a promotion.
Shocking moments: Danny’s death scene; Cottan killing Denton; Cottan taking a bullet for Fleming.
Memorable dialogue: “We gather evidence and the people decide. They’ve decided she didn’t do it because you couldn’t keep it in your pants!” (Kate Fleming to Steve Arnott)
Series four of Line of Duty begins on BBC One on Sunday at 21:00 GMT.