It’s almost upon us; this Sunday night, a brand new episode of Jed Mercurio’s Line Of Duty will batter down our front doors using a police-issue enforcer and spill into our homes shouting indecipherable acronyms into walkie talkies, running up the stairs, kicking over plant pots and occasional tables as they go. 

To help us cope with all the excitement, with major spoilers for series one to four, here’s a wee reminder of the story so far…

Supt. Ted Hastings (Adrian Dunbar)

This is Superintendent Ted Hastings, like the battle, and also unlike the battle in that Ted would never allow himself to be depicted in embroidery by the French. He’s too busy sniffing out bent coppers, saying “God, give me strength” and conducting enquiries to the letter of the law. The letter, which is L.

Other letters are AC, which stand for Anti-Corruption, which is what Ted does and what he is. He runs AC-12, which makes him about as popular as a fart in a lift with the rest of the police service, but Ted pays that no mind. He didn’t come here to make friends. He came here to say ‘fella’ and stand for what’s right.

Helping him are DS Kate Fleming and DS Steve Arnott, whom Ted loves like he’s their Da but God, give him strength sometimes. (Ted’s God is the Catholic one, so as a young Northern Irish officer during the Troubles, he learned about prejudice and police conspiracy the hard way.) They’re good wee officers the both of them but sometimes they need to catch themselves on.

Ted is a plain steak-and-no-sauce kind of man. He’s separated from his wife, childhood sweetheart Roisin, having made an unwise investment and lost their savings, but still wears his wedding ring and lives by his vows. His granite morals often stand in the way of his personal advancement. He’s turned down promotions, sexual advances and white wine, all for the sake of probity.

He’s Ted Hastings, a steadfast island in Line Of Duty’s choppy ethical waters, last seen shooting square between the eyes a member of the ‘Balaclava Gang’ organised crime syndicate attempting to storm AC-12’s headquarters to remove a witness. On you go, son.

Detective Sergeant Kate Fleming (Vicky McClure)

Kate Fleming’s the chameleon of AC-12. A skilled undercover officer, she blends in imperceptibly against any background. Serious Crime, CID, Armed Response… Kate has acted the part in all of them.

‘The part’ is new transfer Kate F, a blue-eyed beauty who’s always ready with a head-tilt and a sympathetic ear. When Kate F isn’t doing pub banter to get in with the lads, she’s approaching her superior officers and saying ‘sorry sir/ma’am, I couldn’t help but overhear. If there’s anything I can do to help with all this corruption you’re up to, just say.’ It’s an approach that works a surprising percentage of the time.

Kate’s a trained firearms officer and gutsier than a butcher’s slab. She won an award for bravery when she tooled up to pursue corrupt officer DI ‘Dot’ Cottan on foot after he tried to escape an AC-12 interview with the help of armed gunmen. Dot ended up taking a bullet for Kate, and while he bled out, she collected his Dying Declaration naming corrupt police names.

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In her personal life, Kate was married and has a son who lives with her ex, and we know her to have had an affair with a married friend, but her head’s really in the job. Though she and Arnott sometimes lock horns and jostle for position, they have each other’s backs. When Cottan attempted to frame Steve, Kate undertook an undercover mission against her own squad and ate a bowl of corrupt chilli to clear Steve’s name. She’s AC-12’s coolest customer.

Detective Sergeant Steve Arnott (Martin Compston)

Aka: the waistcoat warrior. DS Arnott is AC-12’s three-piece-suited pit bull: tenacious, argumentative, cocky and unlikely to let go after he sinks in his jaws. The past four series have put Steve through it. He’s been kidnapped, almost had his fingers amputated, framed for murder and held in custody, beaten with a baseball bat and thrown down a flight of stairs resulting in a coma and temporary wheelchair use. It’s a wonder the man doesn’t start shaking every time he reaches for his lanyard.

Steve was originally recruited by Hastings after he refused to collude in a police cover-up after a botched anti-terrorism operation in which an innocent man was shot dead, proving his moral fibre. He may know right from wrong on the job, but Steve hasn’t always had a squeaky clean personal life. He was romantically involved with a witness on an active investigation, and seduced DI Lindsay Denton in series two in an attempt to uncover the truth about her corruption. (Though the details are sketchy, her audio recording of their liaison, which was played in court, caused Steve major embarrassment and helped to overturn her conviction.) He is unmarried and his on-off girlfriend DS Sam Railston works for murder squad.

Series one

AC-12 exposed ‘laddering’ malpractice by DCI Tony Gates (Lennie James), an award-winning officer whose Serious Crime Unit’s boasted exceptional, and inflated, statistics. Gates’ team included DS Matthew ‘Dot’ Cottan (Craig Parkinson) and DC Nigel Morton (Neil Morrissey), DC Deepak Kapoor (Faraz Ayub), and an undercover DC Fleming. Kapoor was fired from the squad when he was wrongly suspected of leaking information to AC-12 (really, it was undercover officer Fleming, whose cover was blown by DC Morton, who it was revealed had been faking a leg injury in order to exploit desk duty and payments).

While investigating Gates for fiddling the numbers, AC-12 discovered his involvement in a much more serious cover-up. Gates’ wealthy mistress Jackie Laverty (Gina McKee), who was laundering money for an organised crime syndicate her property holdings company, killed her accountant with her car when he discovered her criminal activity. Laverty lied to Gates that she’d been drunk-driving and hit a dog, so he agreed to cover up the incident, staging the theft of her car.

The criminal gang attacked Gates and murdered Laverty, keeping her corpse – contaminated with Gates’ DNA – in a freezer to blackmail him while his squad investigated drug murders they’d committed. Blackmailed, Gates disguised the murders as terrorism-related for the return of Jackie’s body. The gang refused to return it, so Gates tracked down its leader Tommy Hunter, arrested him and elicited a confession while driving, which was broadcast to Arnott and Fleming. Thinking that Hunter had been caught and that, due to his corruption, he faced a lengthy prison sentence, Gates committed suicide by walking into moving traffic, and urged Steve to report that he had died in the line of duty to ensure that his wife and children would receive a pension. Steve and Kate did so.

During the investigation, DS ‘Dot’ Cottan gave (false) evidence that incriminated DCI Gates, which led Supt. Hastings to recruit him for AC-12, thinking him an officer with strong morals. Dot was actually a long-time corrupt officer nicknamed ‘The Caddy’ who’d worked for Tommy Hunter and the criminal gang since he was a teenager. Assistant Chief Constable Derek Hilton (Paul Higgins), later revealed to also be working for the criminal gang, put Cottan forward for promotion to the rank of inspector.

Series two

AKA: did Denton do it? AC-12 investigated DI Lindsay Denton (Keeley Hawes), the sole survivor of an ambush on a police transport in which three officers were killed and a witness severely injured. The ambush had been orchestrated by DI Cottan on the orders of the criminal gang in order to murder former leader Tommy Hunter, then in Witness Protection. Cottan had bribed DS Jayne Akers to place a tracker on Denton’s car, enabling two other corrupt officers (DS Prasad and DS Cole) to follow the convoy and kill Hunter. Akers, who had bribed Denton to collude in the plan to kill Hunter with cash and the promise that she’d be taking a dangerous criminal off the streets, was killed in the ambush, and Hunter was later murdered in hospital by DS Cole. DI Denton was left alive as a scapegoat and because she had no knowledge that Cottan, Prasad and Cole were behind the ambush, her only point of contact having been Akers.

Denton had previously had an affair with her married boss DCC Dryden (Mark Bonnar), which he’d ended when she fell pregnant, pressuring her to terminate the pregnancy. While stalking Dryden, Denton witnessed him engage in sexual activity with a minor – Carly Kirk, a teenager in care who was groomed by DS Cole to work as part of Tommy Hunter’s sex work operation, and who’d been instructed to target Dryden so that Hunter’s gang could gain compromising blackmail evidence on the Deputy Chief Constable. The corpse of a girl believed to be Carly Kirk (not actually her, Cole had disguised another body to look like hers and Carly, unbeknownst to everyone but us, got away) was discovered as part of the gang’s attempt to frame Dryden for the ambush.  

Denton was remanded in custody for her role in the ambush until her trial, where she was abused for being a police officer. Allowed out to attend her mother’s funeral, she was kidnapped by Prasad and Cole, but fought them and escaped. Dot maintained the illusion of innocence by having former colleague Nigel Morton frame dead corrupt officer Jeremy Cole as ‘The Caddy’.

In the finale, DS Prasad gave evidence against Denton in exchange for full immunity from prosecution while DCC Dryden resigned from the police service, and Denton, whose bribe money had been discovered by Steve among her dead mother’s possessions, was sentenced to life for conspiracy to commit murder.

Series three

AC-12 investigated Sergeant Danny Waldron (Daniel Mays), a firearms officer we met shooting dead a suspect who presented no immediate risk, and forcing his team to collude in a cover-up. The question wasn’t whether or not Waldron did it, but why? 

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That was answered when Waldron murdered a second man, the uncle of the first victim. It’s revealed that both men were abusers in a child sexual abuse ring that operated for years at Sands View Children’s home, where Waldron grew up. Soon after, Waldron was shot during an operation and dies. His team first said he shot himself, but later placed the blame on PC Rod Kennedy (Will Mellor) after he was found hanged (by members of Cottan’s organised crime syndicate, to frame compromised officer PC Hari Bains, whom they were also blackmailing). Before Waldron died, he’dleft a list of the names of the Sands View abusers (which included former gangleader Tommy Hunter) addressed to Steve Arnott, but the list was intercepted and destroyed by Cottan.

The main action of series three was split between the attempt to untangle the Waldron case, Cottan’s attempt to frame Arnott as ‘The Caddy’ – he planted evidence and attempted to prove that Steve had killed PC Kennedy – and the return of Lindsay Denton as she fought to have her life sentence overturned by exposing Arnott’s dubious honeypot operation and arguing that he planted money Akers bribed her with. Denton was freed, and briefly teamed up with Arnott to discover a digital copy of Waldron’s abuser list, which they found to include now-retired senior police officer Chief Inspector Fairbanks (with whom Supt. Hastings was seen to share a Masonic handshake). Cottan, using Arnott’s car, attempted to bribe Lindsay in exchange for the abuser list linking Tommy Hunter to Sands View, but she refused, so he shot her dead and framed Steve for her murder.

Series four

Forensics bod Tim Ifield (Jason Watkins) urged DCI Roz Huntley (Thandie Newton) to consider forensic evidence that he thought showed Michael Farmer, the suspect she just charged for a series of murders, was framed. When, under pressure from her boss ACC Hilton, Huntley ignored him, Ifield took the matter to AC-12, suspecting a cover-up. Huntley confronted Ifield at his home about the AC-12 investigation and in a struggle, she was accidentally knocked unconscious.

When Huntley awoke, she saw Ifield about to dismember her presumed-dead body with a chainsaw, which then accidentally killed him in the ensuing fight. Huntley covered up Tim’s death by tampering with forensic evidence that would have put her at the crime scene. She amputated three of Ifield’s fingers because in a dying act, he’d scratched her arm to get her DNA under his nails. She buried the fingers along with Ifield’s laptop in woodland. The resultant arm wound became infected and worsened throughout the series, until her lower arm had to be amputated. Seeking treatment for the infected wound was key in the discovery of Huntley’s cover-up.

To save herself, Huntley attempted to frame her husband (Lee Ingleby) for Ifield’s murder. His university friend and lawyer Jimmy Lakewell (Patrick Baladi), was revealed to have been working with the organised criminal gang responsible for the original murders all along. Lakewell had suggested Farmer, a former client of his and a vulnerable young man, as a potential candidate to be framed for the killings, which had been committed to gain blackmail evidence on police officers (the victims had worked in the sex trade and their corpses were contaminated with DNA from officers).

Confronting Huntley’s husband and Lakewell, DS Arnott was brutally attacked with a baseball bat and pushed down several flights of stairs by a member of the criminal gang, injuring him gravely. Fleming eventually discovered Huntley’s guilt by tracking her movements on the night of Ifield’s murder and discovering buried evidence in the same spot in which Huntley had discovered a body on a previous case.

When accused, Huntley requested Jimmy Lakewell as her solicitor, admitted to Ifield’s accidental murder and subsequent cover-up, exonerated her husband, and, because she was still good police, arrested Lakewell for his part in the criminal conspiracy and convinced newly corrupted officer Jamie Desford, who’d been taken under the wing of ACC Hilton (bent as they come) to stop before it was too late. Before Hilton died (apparently taking his own life but much more likely murdered by the criminal organisation blackmailing him), he was being passed information about AC-12 by a member of its own team, PC Maneet Bindra, who’d also tricked Desford into giving her his login details so she could access sensitive files at Hilton’s request. Huntley got ten years. Lakewell pleaded guilty and refused to testify and enter witness protection, presumably knowing there’d be no point seeing as what happened to Tommy Hunter in series two.

And AC-12 lived to fight another day. Specifically, this Sunday. And then five more Sundays after that one. And then another six after that. Thank goodness.

Line Of Duty series 5 starts on Sunday the 31st of March at 9pm on BBC One.