Who is the final ‘H’ in Line of Duty? All the suspects, clues and theories

The finale followed weeks of cryptic clues, crazy fan theories and all sorts of wild guesses, and we've run through the whole backstory below, profiling not just the man who turned out to be guilty but all those who had previously been in the frame as well.

Read on for all the information about who was revealed as H, how that was ultimately explained, and what that means for all the other runners and riders who were previously under suspicion.

Who is H, or the Fourth Man?

The first time "H" came onto our radar was at the very end of season three, when one story arc came to an end – and another began. AC-12 had finally found "The Caddy", but now The Caddy (unmasked as DI Dot Cottan) revealed in his "dying declaration" that there was someone called "H" who was the real big bad.

Or at least, that's what we thought he meant at the time. Because while it was initially assumed that Dot was giving the first letter of a surname (like Hastings, Hargreaves, Hilton or Huntley), or a codename, it turned out to be far more complex than that.

Here comes the complication! At the end of season five, AC-12 re-examined Dot’s dying declaration – and came up with a new theory. By using his left hand to tap out “dot dot dot dot” (‘H’) in Morse Code, Dot was apparently trying to tell Kate that there were four "Dots" - i.e. four “Caddies”. AC-12 identified these as Dot Cottan, Gill Biggeloe, Derek Hilton and one more senior person within the police force, identity unknown. That person became known as the "Fourth Man".

But by the season six finale, the "four Dots" explanation was falling apart. And when DCI Ian Buckells (Nigel Boyle) was identified as the senior corrupt cop who'd been calling the shots and sending all those instant messages, he made it clear that though he was the man who AC-12 had been looking for, things were a lot more fluid than AC-12's working four Dots theory would suggest.

As he explained, from the '90s onwards it was criminal boss Tommy Hunter who was top dog in this corrupt network, until his arrest in 2012 and death in 2014 – after which the OCGs splintered into multiple groups without clear, steady leadership.

In his time, Tommy had worked closely with Fairbank and Thurwell within the police force. After they retired, Hilton and Dot took over their roles, and after those two were killed, Buckells – by now senior enough to do some serious damage – became the point-man within the police, now working with disparate OCG groups instead of a Tommy-like ultimate criminal leader. Buckells didn't even mention Gill Biggeloe, who Ted Hastings had assumed was an H.

So even with Buckells behind bars (at least for now), there could still be others out there. Perhaps young, corrupt officers working their way up the ranks from the bottom, as Ryan Pilkington had intended to do.

Is Ian Buckells H?

Yes, it's Buckells! Were you expecting that? We certainly weren't.

Buckells has been part of Line of Duty since season one, and has always come off as a man who'd been promoted beyond his abilities, and who aims to do the minimal amount of work possible to get the job done. After his initial AC-12 interview, he seemed genuinely bewildered about how he'd been so skilfully stitched up by Jo Davidson. While Jimmy Lakewell was murdered in front of him as an apparent attempt to scare him, he shook so much he spilled the milk for his tea. Hardly criminal mastermind material!

But there have been some signs throughout Line of Duty that Buckells was bent – and, more recently, that he was actually pulling the strings.

In season one, Buckells was the one who let DS Matthew "Dot" Cottan (Craig Parkinson) – later revealed as "The Caddy" – speak to Tommy Hunter in private after his arrest, letting him into the back of the van for a chat. In season four, he continued to be antagonistic towards Kate and AC-12. And by season six, he had managed to emerge (despite his mediocrity) as a Detective Chief Inspector at Hillside Lane, rocketing upwards through the ranks.

Buckells was the one who strongly requested DCI Jo Davidson as a new SIO on the case – it turned out the OCG/police alliance had been coercing her into compliance for years, so he could easily give her orders via instant message and control the direction of Operation Lighthouse. Not that Jo knew it was Buckells who was messaging her. As far as she was aware, it was still Fairbank pulling the strings from his jail cell.

Knowing that he needed to close the case and steer attention away from organised crime, Buckells put pressure on Jo to charge Terry Boyle (Tommy Jessop) with Gail Vella's murder. His ex-lover Deborah even gave a false witness statement to help. But Jo wouldn't do it, and managed to frame Buckells as a corrupt cop instead (which is objectively quite funny, as she didn't know he was H at the time).

While his lawyers fought to spring Buckells from prison, the police crime boss simply carried on his H duties from his cell using a secret laptop. He ordered Jo to "get rid" of Kate (with Ryan's murderous help), and he pushed for the OCG's "assets" in Brentiss Prison (Merchant and Leland) to arrange for Jo to be killed too.

Why was he doing all this? Well, when he was just DC Buckells in 2003, he worked with senior corrupt policeman Marcus Thurwell, taking part in the investigation into the death of Lawrence Christopher – which was deliberately botched because one of the killers was Tommy's son, Darren Hunter. As journalist Gail Vella (Andi Osho) got closer and closer to the truth, Buckells saw the risk – and got the OCG to assassinate her.

Aside from that, Buckells has apparently had a fruitful career as a corrupt police officer. He coordinated and okay'd the Eastfield Depot raid, and passed on information whenever it was needed for decades. His motive? Money. Which funded a secretly lavish lifestyle.

Is Philip Osborne H?

Chief Constable Philip Osborne (Owen Teale) has always been a shadowy figure in the background of Line of Duty, appearing in season one – and later reemerging in season six as a central character, even though he never appeared 'in the flesh' (it's all been archive material and news footage). And while he didn't turn out to be H by the end of season six, we're still very suspicious about him.

We know that Osborne is the mastermind behind this new move to merge AC-3, AC-9, and AC-12, while cutting the combined anti-corruption unit's staff by 90 per cent. He's the one who's been forcing through Ted Hastings' retirement. And he's the one who secretly put trackers on personal vehicles of AC-12, so he could always know what they were up to if he wants to.

Also, Hastings was meant to have a month until his retirement. But then Osborne jumped the gun and decided to bring AC-12 to its knees even sooner – appointing AC-3 boss Patricia Carmichael to take charge, and pulling funding for all surveillance operations. That put Kate Fleming and Terry Boyle in immediate danger, and freed Jo Davidson and Ryan Pilkington up to do whatever they liked. It also meant that, in Jo Davidson's interrogation, his favourite lackey Carmichael could steer the questioning away from anything about wider police corruption - and any questions about Osborne's own role.

He's also currently on a crusade to push back on any oversight of the police, giving an impassioned speech about "false allegations of corruption" against police officers, declaring: "We must defend this constabulary from those who would obstruct us in serving the public. Not only does this force face enemies without, there are enemies within. I will personally see to it that those enemies within are made to suffer the consequences."

Line of Duty fans have known since season one that Osborne is a villain: when his men killed an innocent man in a botched anti-terror operation by going to the wrong flat, he orchestrated a cover-up and cast Steve Arnott (Martin Compston) out of the fold when he refused to go along with it.

But season six suggested he's not just villainous, but potentially corrupt – and working with the OCG. Back in 2003, he was one of the police officers working on the Lawrence Christopher case. What if he was in league with Buckells in ordering the Gail Vella murder? After all, both he and Buckells had a lot to lose if the truth came out.

Is Jo Davidson H?

By the end of season six, we knew for sure that Acting DSU (formerly DCI) Jo Davidson had spent her whole career being ordered around by the OCG and corrupt cops – and she hated it. In some ways you could consider her an H, as (like Dot Cottan) she went into the police force to do Tommy Hunter's bidding; but she was never the one giving orders, only receiving them.

As AC-12 worked out, Jo is related to Line of Duty's original "big bad", crime boss Tommy Hunter, who once led the OCG. More specifically, she is both his niece and daughter (!), as DNA analysis of "runs of homozygosity" has established. Jo didn't know "Uncle Tommy" was also her biological father until Carmichael told her during her interrogation at AC-12, and she was genuinely devastated.

Jo says she was born after her mother Samantha Davidson was raped at the age of 15. She grew up in Glasgow – away from the criminal Hunter family – and her future looked promising. That is, until Tommy came knocking and ordered her to join the police force as soon as she left school, so she could act as the OCG's "inside woman". Her distraught mother then killed herself, and Tommy subsequently fed Jo a convincing lie about bent copper Patrick Fairbank being her biological father (and therefore her mother's rapist).

Jo became a police officer, and for her entire career she has been forced to do the bidding of the OCG and/or a senior corrupt police officer. With every deed she committed, the OCG had more evidence to use against her if she tried to break away.

Under orders, Jo worked hard to lead the Operation Lighthouse investigation away from organised crime, and to hide her own complicity. That included tipping off the OCG when a call came in about "Ross Turner" (Carl Banks); diverting the armed convoy; planting burner phones in Farida's house; framing DI Ian Buckells; and leading Ryan to a meeting with DI Kate Fleming (Vicky McClure) so he could execute her. However, she also claimed that she'd been trying to find a way out – and that, at heart, she's not "bent". She couldn't bring herself to charge Terry Boyle.

Jo Davidson is now in witness protection, and her whereabouts is unknown.

Is Marcus Thurwell H?

Marcus Thurwell (James Nesbitt) was a bit of a red herring. Sure, he was an H in his time (in that he was a senior corrupt cop working with the OCG), but he wasn't the person AC-12 was really looking for.

Thurwell had been SIO on the Lawrence Christopher case (and cover-up), and he was also involved in the Sands View Boys' Home case. The only reason he wasn't brought in for questioning by AC-12 during the events of season three was that he'd already retired to Spain, and the investigation prematurely ended after Fairbank's arrest and conviction. He seemed of more minor interest, anyway.

But during season six, Thurwell suddenly became a key figure, despite never being seen in person (at least not alive). Was he H? Was he the person Jo Davidson believed to be her father? If H's instant messages were coming from an IP address in Spain, was that Thurwell calling the shots from his Spanish villa during his "retirement"?

AC-12 asked the Spanish police to find him, and find him they did – only he was dead, strangled, and decomposing on the floor. At least they were able to seize communications equipment from Thurwell's home, proving that he was helping Buckells out by routing his instant messages through the Spanish IP address and back to various criminals and cops in the UK.

Buckells says Thurwell used to be one of the top dogs in the network, but then passed the mantle on after taking early retirement. That seems very plausible. But the question remains: who killed him, and why?

Is Patricia Carmichael H?

DCS Patricia Carmichael (Anna Maxwell Martin) isn't H, but she is also suspiciously keen to downplay police corruption.

The AC-3 boss was brought in to investigate Ted Hastings in season five, and was ruthlessly determined to prove he was H – although just when she thought she had won her case completely fell apart. She then returned in season six to take over AC-12 ahead of the Chief Constable's upcoming merger, taking pleasure in cancelling the team's ongoing surveillance operations (which were deemed too expensive) and frustrating AC-12's progress.

But is her desire to bring down Hastings and AC-12 motivated by malice – after they made her look foolish in season five? Is she driven purely by ambition, which is why she's carrying out the Chief Constable's orders without question, just proud of herself for getting a promotion? Or is she a bent copper?

Her interrogation of Jo Davidson hardly got us any closer to finding an answer. To recap: every time Ted Hastings or Steve Arnott tried to ask a question about the identity of "H" or links to wider police corruption, Carmichael jumped straight in to say, "I think we'll leave it there" or "I would prefer to confine myself to the set parameters of the anti-corruption enquiry."

Is Kate Fleming H?

After the trailer dropped for season six, there were increasingly loud voices wondering whether DI Kate Fleming (Vicky McClure) was "H". It would certainly have been a twist: the anti-corruption crusader and dedicated undercover officer, who is actually running the whole show. No wonder AC-12 had never been able to unravel the whole network of corrupt police officers – the key was hiding in plain sight!

And we were definitely confused by that car chase, in which Kate inexplicably fled the scene of Ryan's shooting – with Jo Davidson in tow. What was her plan here? Where were they going? Why did she trust Jo enough to let her keep her gun, despite the fact that Jo had just tried to lure her to her death? Why did she let Jo take the blame for killing Ryan? So many questions, such confusing behaviour.

None of that pointed clearly to Kate being "H", however. For one thing: why would she get a dying declaration off Dot Cottan if she knew it would incriminate her? And if she did, why would he use morse code instead of just... pointing at her?

No, Kate was never going to be H.

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Is Ted Hastings H?

AC-12’s beloved crusader against police corruption, Superintendent Ted Hastings, had a shocker in season five – and at one point, it even looked like he’d be going to jail as Carmichael built a solid case against him. Ultimately, his “best team” came to his rescue and presented new evidence, clearing his name. But some things are still unexplained, even at the end of season six.

We're still puzzling over this: when messaging John Corbett and Lisa McQueen, “unknown user” (H) spelled “definitely” with an “a”. Later, Ted hijacked a conversation AC-12 were attempting to simulate between H and the OCG via the same messaging system, writing that he could “definately pull the right strings”. In his interrogation, Hastings claimed he’d consciously mimicked the mis-spelling, which is not a fully believable answer. But all the other "definately" clues pointed to Buckells, so we can only assume Hastings' "definately" was a red herring at this point.

Also, we’re still doubtful about Ted Hastings’ real reasons for turning off his phone in season five so he was un-trackable, wrapping his laptop in bubble wrap, and taxi-ing to an electronics disposal shop to get it professionally destroyed. Hastings’ eventual confession that he was “looking at pornography” doesn’t really stack up, especially as we’ve seen his laptop open in his hotel room with a line of text popping up – just like the messages ‘H’ used to communicate with the OCG. If the porn was “nothing illegal, nothing extreme”, why go to such lengths to destroy the entire machine unless there was something else incriminating on there?

And then there's the troubling fact about John Corbett, which he's finally admitted to Kate, Steve and Carmichael. In season five, Hastings visited Lee Banks (Alastair Natkiel) at Blackthorn Prison and told him there was a rat in the OCG; the OCG soon worked out that rat was Corbett – and killed him. However, Hastings says he was a) trying to get Corbett to take ‘refuge in a police station’ by blowing his cover and forcing him off the job, and b) possibly influenced by Corbett's involvement in Maneet's murder and the assault on Roisin Hastings. He also felt terrible afterwards, giving £50k to Steph Corbett under the radar.

It’s possible Hastings is not as straight-laced as he’d like everyone to believe. But in season six, Hastings didn't turn out to be H, and it's hard to even consider him for the title "bent copper" when he's been working so hard to uncover police corruption.

Is Rohan Sindwhani H?

Police and Crime Commissioner Rohan Sindwhani (Ace Bhatti) was a new addition to Line of Duty in season five, and has now quit his job in season six. But despite his early promises to fight police corruption, in reality he spent his entire tenure sweeping things under the carpet, and trying to keep AC-12 in check. Why? Was he simply trying to protect the reputation of the force, or was he up to something more sinister?

After all, Sindwhani was the one who gave this statement, despite everything he knew about Gill Biggeloe and PS Tina Tranter and co: “The Deputy Chief Constable and I are pleased to report Operation Pear Tree has completed a thorough investigation into institutionalised complicity between organised criminals and corrupt police officers. Its robust findings couldn’t be clearer. There is no institutionalised corruption in this police force.”

But whatever his motives, it seems unlikely that Sindwhani is "H", for several reasons. One: he has now stepped down as PCC after clashing with the Chief Constable, which "H" would be unlikely to do. Two: he let Gill Biggeloe manipulate and push him around. Three: he was only elected as PCC after Dot Cottan gave that (in)famous dying declaration, so he's unlikely to have even been on Dot's radar.

Is Andrea Wise H?

Detective Chief Constable Andrea Wise (Elizabeth Rider) joined Line of Duty in season five, and from the very first minute she was reluctant to support AC-12’s investigations. She repeatedly obstructed AC-12's access to Operation Pear Tree (perhaps to avoid them getting close to the truth?) and also ordered a separate inquiry into Hastings (perhaps in a bid to frame him as "H"?).

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Then, in season six, she did her best to scupper AC-12's investigation into Operation Lighthouse – even arranging things so that the first raid on Hillside Lane had to be aborted, and giving Jo Davidson enough time to get rid of important files. And she took great delight in announcing to Hastings that he'd have to retire.

But like Sindwhani, DCC Wise's main objective seems to be to sweep police corruption under the rug by painting the issue as a few isolated examples rather than a institutional problem. So DCC Wise could be "H", but it's more likely that she's either just averse to bad publicity – or under the thumb of someone more senior, like the Chief Constable.

If you've now seen the series 6 finale, we have plenty of coverage - you can read our Line of Duty ending explainer, check out the Line of Duty unanswered questions left up in the air, or take a look at all of the Line of Duty red herrings that teased and deceived the finale outcome.

Line of Duty season 6 is available on BBC iPlayer now, and we have all the latest news on Line of Duty season 7. Check out the rest of our Drama coverage, or take a look at our TV Guide to find out what's on tonight.