Line of Duty: All you need to know to get up to speed for series six

Line of Duty, BBC One's hit drama about police corruption, returns to our screens on Sunday 21 March.

The sixth series will introduce Kelly Macdonald as Detective Chief Inspector Joanne Davidson, the senior investigating officer of an unsolved murder.

• Kelly Macdonald joins Line of Duty cast

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.

Series six sees the AC-12 team joined by DC Chloe Bishop (Shalom Brune-Franklin)

But if you haven't watched it before or need a reminder of the plot, look no further - here's a handy bluffer's guide to series one to five.

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)

The first series saw Tony Gates (Lennie James) investigated by Arnott and Fleming

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 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)

The second series saw DI Lindsay Denton (Keeley Hawes) come under suspicion

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 shooting involving Danny Waldron (Daniel Mays) kicked off the third season

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 (2016)

Thandie Newton took centre stage in series four

Under pressure to catch a serial killer, DCI Roz Huntley could do without forensic coordinator Tim Ifield (Jason Watkins) questioning the evidence against the suspect.

He takes his concerns to AC-12 and Kate Fleming goes undercover to investigate Huntley. Suspecting Ifield has shopped her, Huntley confronts him at his home.

Huntley returns to work with a concealed hand injury. Ifield is then discovered dead, with three fingers amputated.

Taking charge of the crime scene, Huntley implicates Ifield in another murder. Her husband refuses to confirm her alibi and falls under suspicion himself. A man in a balaclava throws Steve Arnott down a flight of stairs.

Huntley has to have her hand amputated and is arrested for killing Tim Ifield. Her solicitor is found to be linked to "Balaclava Man" - as is Huntley's boss, Assistant Chief Constable Derek Hilton.

A recording of Cottan's deathbed declaration reveals he was promoted by a corrupt copper whose name begins with H. Is this the newly deceased Hilton... or could it be Ted Hastings?

Shocking moments: Huntley and Ifield's fight; Arnott being attacked.

Memorable dialogue: "This is beginning to feel like a life's work!" (Ted Hastings)

SERIES FIVE (2019)

Stephen Graham is revealed to be "Balaclava Man" in series five

When a police convoy is hijacked by an organised crime group (OCG), AC-12 suspect undercover police are involved.

They're right. The raid was masterminded by DS John Corbett (Stephen Graham), who has been 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).

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, McQueen given immunity and Ryan Pilkington, Corbett's killer, inducted as a student police officer.

Shocking moments: Corbett's murder; Biggeloe being attacked.

Memorable dialogue: "I'm just trying to get to the truth. The top brass, they don't want me to succeed!" (John Corbett)

Line of Duty is on BBC One at 21:00 GMT on Sunday 21 Marc