From Tony Gates to Roz Huntley: how are all four series of Line of Duty linked together?

But how does it all fit together? The final scenes of episode six saw boards erected in AC-12 featuring pictures of all the involved parties – "this is beginning to feel like a life's work," mused Ted Hastings as he took in the scale of the nefarious underworld he was facing.

So, who is on the board?

A bit hazy on the who's who and how they're all connected? Let us help break it all down for you...

1. Detective Chief Inspector Tony Gates (played by Lennie James)

Series: one
Status: deceased

Tony was the force's golden boy –- a lead investigator and Officer of the Year. But he was having an affair with local property developer Jackie Laverty who orchestrated a hit on her accountant once she realised he was digging around in her money laundering schemes. Gates helped her cover it up but when men clad in balaclavas (yes, BALACLAVAS) burst into her home, killed Jackie and placed his fingerprints all over the murder weapon, he was beholden to their demands. The man holding Gates on such a tight leash? Tommy Hunter whose lackeys showed the copper her frozen body and threatened to dump it, covered in his fingerprints, if he didn't toe the line.

With the vice tightening – and AC-12 on his tale – Gates saw no way out and committed suicide, ensuring his family received a death-in-service payout. But not before he helped Steve Arnott apprehend Hunter who cooperated with investigations in exchange for immunity.

2. Detective Inspector Lindsay Denton (played by Keeley Hawes)

Series: two and three
Status: deceased

Denton was the main antagonist of series two, investigated by AC-12 for her role in an ambush that killed Tommy Hunter (when he was in a police witness protection programme) and officer Jayne Akers. Innocent party or guilty colluder? That was the question Hastings & co faced, eventually convicting Denton of conspiracy to murder for which she received a life sentence.

But they only had part of the story. Denton had been recruited to a plan to hand Hunter over to criminals keen to silence him – she was persuaded to partake by Akers who told her it would save the life of 15-year-old Carly Kirk. Denton had been paid handsomely for her involvement but had no idea of the murderous turn the ambush would take – and how she'd be framed for more than her part in it.

Her story appeared to end with her prison sentence but in series three Denton resurfaced, overturning her conviction and playing an instrumental role in identifying corrupt AC-12 copper Matthew "Dot" Cottan as the Caddy – an officer working for Hunter, nicknamed from his childhood years caddying for the criminal.

Denton's own investigation saw her uncover a list made by Danny Waldron (see below) of a paedophile ring that preyed on him and other inhabitants of a children's home. The list named a string of Dot's associates which the bent copper tried to prevent Denton from emailing to AC-12. As she hit send, he shot her in the head, attempting to frame her death as the work of AC-12 officer Steve Arnott.

3. Sergeant Danny Waldron (played by Danny Mays)

Series: three
Status: deceased

Waldron was billed as the officer to be investigated by AC-12 in series three. The opening scenes saw him shoot dead a criminal, Ronan Murphy, in cold blood before torturing and killing Linus Murphy, one of the mourners at his funeral. But Waldron's time was cut short when he was involved in a scuffle with his squad at the end of episode two and ended up fatally wounded.

Before his death, he'd left a list of sexual abusers who visited Sandsview children's home where he grew up – a register containing the names of Ronan, Linus, Tommy Hunter and more. It was addressed to Steve but found its way into the hands of corrupt Dot who destroyed it. And the trail would have gone cold if it wasn't for Denton and her efforts to recover the evidence and put it in the hands of AC-12, helping to bring to justice the abusers who Waldron had named.

4. Jackie Laverty (played by Gina McKee)

Series: one
Status: deceased

Laverty was a property developer and the lover of Tony Gates – although it soon transpired that her business was a cover for a money laundering outfit connected to Tommy Hunter. She bumped off her accountant when he started sniffing around her dodgy dealings, convincing Gates to help her cover it up. But events took a dramatic turn when Laverty was murdered by Hunter's associates and her frozen body was placed in storage.

It's never been recovered and her killing remains unsolved by the police – although Gates told Arnott he had seen the body in a freezer when held hostage by Hunter. It was that admission that allowed AC-12 to investigate Leonie Collersdale's murder in series four – her body was also kept in cold storage and the connection between her killing and Laverty's gave Hastings the ability to dig into a case he'd been banned by Assistant Chief Constable Derek Hilton from investigating.

5. Detective Inspector Matthew "Dot" Cottan (played Craig Parkinson)

Series: one to three
Status: deceased

Dot spent the best part of three series concealing the fact that he was the mysterious "Caddy" – an asset planted at the heart of the police force by Tommy Hunter. He was first seen in series one working for Gates only to be revealed to viewers as Hunter's inside man during the series finale when he urged his boss to go into witness protection. But Dot betrayed Hunter in series two when he arranged his execution (to protect his own identity). With Ted Hastings none the wiser, he was permanently transferred to AC-12 at the end of series two where he was – ironically – tasked with identifying the Caddy.

But series three saw the net tighten on Dot as he desperately sought to keep Waldron's list of sexual abusers from the eyes of AC-12. He tried to frame Steve as the Caddy – also attempting to pin Denton's murder on his colleague – but cracks began to appear in his facade as AC-12 called him in for questioning.

In the face of damning evidence, Dot enacted his exit strategy and escaped the interview room in dramatic fashion. He went on the run with Kate in hot pursuit but when a mystery assailant attempted to shoot her, Dot jumped in the path of the bullets and saved her life. His dying declaration implicated Superintendent Patrick Fairbank in the paedophile ring that operated at Sandsview.

Dot also named a police officer – with a name beginning with "H" – still operating at the heart of the force at the behest of this mysterious network of criminals. That officer was identified as ACC Hilton in the series four finale.

6. Tommy Hunter (played by Brian McCardie)

Series: one and two
Status: deceased

Tommy Hunter was a bad, bad man. He held the noose around Gates' neck, he was seemingly the one who ordered the hit on Jackie Laverty and he was responsible for trying to take out Steve for digging too deep into his criminal connections. But Hunter didn't bank on Gates doing the right thing – before his death, the police officer led Steve to his blackmailer in the series one finale.

Under arrest and with his back against the wall, Hunter – advised by Dot – bought his immunity with information and entered a witness protection scheme. The start of series two saw the ambush Denton witnessed end in the death of both Hunter and his protection officer Akers. Whoever is top of the criminal tree clearly didn't want to risk him leaking information...

7. Detective Chief Constable Mike Dryden (played by Mark Bonnar)

Series: two
Status: alive

A police big dog, Dryden came under suspicion from AC-12 in series two. He was initially caught up in a furore around a speeding ticket when a story surfaced in the press that he'd taken points for his wife. But that wasn't the limit to married Dryden's troubles – he'd had an affair with Lindsay Denton in the months leading up to the Tommy Hunter ambush and it had ended badly.

Denton informed AC-12 that she thought Dryden was setting her up and they began investigating, finding photos of the Detective Chief Constable with Manish Prasad – a known corrupt officer – and separate snaps of the police chief receiving oral sex from 15-year-old Carly Kirk, known to be working for Hunter and the teenager Denton was hoping to save.

Dryden claimed he was being set up and with Denton's conviction he was never exposed as having had sexual activity with a minor. But he did resign from the force and received a suspended sentence for perverting the course of justice thanks to his speeding ticket offence.

8. Detective Sergeant Jeremy Cole (played by Henry Pettigrew)

Series: two
Status: deceased

Cole was a nasty piece of work – one of two corrupt detectives in vice squad secretly working for the criminal conspirators. He ambushed Tommy Hunter and Jayne Akers before heading to the hospital where Hunter was being treated to finish him off. You may also remember him as the hitman disguised as a nurse who chucked Jessica Raine's DC Georgia Trotman out of a window at the start of series two.

Cole also helped to kidnap Lindsay Denton from custody but was eventually killed by his partner in crime, Manish Prasad, who deemed him a liability.

9. Former Chief Superintendent Patrick Fairbank (played by George Costigan)

Series: three
Status: alive

Fairbank is the retired Chief Superintendent of vice squad and an old buddy of Ted Hastings. He was implicated in the investigation into the paedophile ring, run by Hunter at Sandsview children's home, when he was named in the list Danny Waldron left for Steve – the list that also cited Ronan and Linus Murphy. Fairbank denied prior knowledge but the testimony of Joe Nash, who also grew up at the home, confirmed that he'd not just looked the other way – he'd also abused the boys living there. Fairbank was convicted on evidence given by Dot in his dying declaration and is currently serving ten years in prison.

10. Detective Sergeant Manish Prasad (played by Sacha Dhawan)

Series: two
Status: alive

Prasad was working with Jeremy Cole on vice squad and was carrying out the dealings of the criminal gang when the pair executed Hunter and Akers. But his connections to the underworld ran deep – he knew Dryden from his days heading up vice and was familiar with his penchant for younger girls. So he recruited 15-year-old Carly and served her up to seduce Dryden so Tommy Hunter could blackmail him.

Unfortunately for Prasad, he'd underestimated Lindsay Denton who, after being kidnapped by Cole and Prasad, rammed a car into the latter and forced him to record his confession. The bent copper was yet another criminal who exchanged information for immunity – this time playing a part in convicting Denton for a crime he knew she hadn't committed.

Who is "H"?

Of the three boards erected in AC-12, one examined all the officers who could be "H" – the corrupt senior policeman named in Dot's dying declaration. There are some familiar faces there including Hilton who we've got to assume is the very "H" Dot implicated. The fact he made a run for it when Roz Huntley had proven his guilt speaks volumes.

Ted seems satisfied that Hilton was their man but there are some other options – Roz is on there, although the idea of her being embedded at the top of this criminal network seems a little far-fetched given the events of the series finale.

Hastings appears on the board – a prospect we can't even begin to consider – and there's also Lester Hargreaves, Chief Superintendent of murder squad who was a favourite of Dryden and who we wouldn't trust as far as we could throw him.

And what of those five new faces: Michael Hill, Susan Hyde Albert, Paul Haleton, Raymond Hall and Michelle Harris. Will we see more of them in the next series?

Who are the victims?

The final board looks at the female victims – women whose bodies are used by the criminal network to blackmail police officers. There's Carly King – who viewers are aware escaped the clutches of Tommy Hunter, although the police still have her down as missing. Baswinder Kaur and Leonie Collersdale are the two girls murdered by a balaclava man, while Hana Reznekova was abducted and then rescued by Roz Huntley.

Rikki Neville is the missing girl Kate purported to be investigating when she started working undercover at Roz's station Polk Avenue.

And added to this board is one of the balaclava men – the assailant who killed Baswinder and Leonie, who kidnapped Hana and who attempted to orchestrate the evacuation of Jimmy Lakewell before Ted Hastings shot him dead.

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So how does this big conspiracy fit together?

Well, that's the million dollar question. One criminal network stretching across four series with victims (Danny Waldron), blackmailed officers (Tony Gates, ACC Hilton), good coppers forced to desperate lengths (Lindsay Denton, Roz Huntley) and devious criminals (Tommy Hunter, Dot Cottan).

We are yet to meet the man or woman pulling all the strings – will series five see their identity finally revealed...?

This article was originally published on 30 April 2017