Who is H in Line of Duty? We rate the most popular theories - from Philip Osborne to Kate Fleming

The compelling narrative interwoven through six seasons could come to end on Sunday (C) World Productions - Photographer: Steffan Hill

Are audiences really about to find out who ‘H’ is? Well, according to the finale trailer released earlier this week, the mystery might finally be revealed.

But it does all seem a little too good to be true.

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Line of Duty fans have justified reason to think that, thanks to show creator Jed Mercurio’s history of twist endings. But season six has been the closest AC-12 have got to discovering the truth behind the mysterious ‘H’.

The compelling narrative interwoven through six seasons could come to end, so here is a look at all the possible theories behind the letter that has left UK viewers scratching their heads.

Is Hastings the man at the top?

No character in Line of Duty has been so beloved and hated as Superintendent Hastings (Adrian Dunbar).

He loves catching ‘bent coppers’ and he loves the ‘LETTER of the law’. But it wasn’t until the camera started focussing on him in season four that suspicion began to arise.

Far from flawless, it was revealed he’s had financial trouble (often a gateway into helping the OCG), and his old school wordplay and male centric office structure has seen him targeted for allegations of sexism.

The top piece of evidence against the gaffer is his misspelling of the word “definitely”. The misspelling has appeared in multiple episodes, and was last seen in season six, episode five, where DCI Jo Davidson (Kelly MacDonald) is seen sending messages to an unknown person via a computer.

After getting asked to “get rid” of DI Kate Fleming (Vicky McClure), Davidson asks if this would be her last job, and the unknown person replied, “Definately”.

Hastings put himself in the mix when impersonating communications via computer from ‘H’ to the OCG, and misspelled the word “definately”. The typo was raised by DCI Patricia Carmichael - but Hastings glanced over it, saying he had studied the typing style of ‘H’ “quite closely”.

Iain’s opinion:

I can’t help but think we are being led down the wrong direction in believing our beloved Ted Hastings is ‘Person H’. Handing over the £50,000 under the table to John Corbett’s widow was a shady move, but I don’t think he’s got it in him to be the head of the OCG.

I want to know why Hastings got involved in Anti-Corruption. His days in Northern Ireland have been teased, but my bet is he had a run-in with corrupt officers, did some dodgy dealings, hated the lifestyle, and has now devoted his life to catching the top man. He’s far from perfect, but he doesn’t have the heart to order merciless miscarriages of justice.

Is Carmicahel is the ‘fourth wo-man’?

Loved for her flawless performance. Hated for her snide, Professor Umbridge-style remarks.

But after reappearing in season six, Line of Duty fans have become determined to prove Patricia Carmichael (Anna Maxwell Martin) is ‘H’. Even by looking at the tapping of her pen

Fan theories on Carmichael include tapping morse code of the letter H through her pen to intimidate Jo Davidson, and lawyer Jimmy Lakewell advising Steve Arnott (Martin Compston) to “look beyond the race claim” - take the letters in race claim away from Carmichael and it equals H.

Line of Duty fans amaze me sometimes.

However, the evidence is beginning to stack up against Carmichael, most crucially, her constant dismissal of any questioning into organised OCG links to top level corruption in the police force. Does she despise Hastings that much, or is she attempting to steer away from the truth?

Iain’ opinion:

I can’t help but feel Carmichael is part of the larger corrupt network, and is yet another pawn on the OCG’s chessboard. Her character represents a hatred of the old guard, Hastings included, and she will stop at nothing to accomplish her goals.

Does that make her person ‘H’? No. But I do think she will do all she can to stay in the Chief Constable’s good books.

Is Philip Osborne the most obvious candidate?

After making a short but memorable appearance in the opening episode of Line of Duty, Chief Constable Philip Osborne (Owen Teale) has emerged as a prime suspect for person ‘H’.

He is attempting to break up the anti-corruption units, and it has been revealed he had motive for Gail Vella’s murder. The character also played a role in the institutional racism behind the death of Lawrence Christopher. A truly contemptible character, all told.

We know he’s corrupt, but is he the top man? The finale definitely needs to tie up some loose ends regarding his story arc.

Iain’s opinion:

We already know he is corrupt (season one, episode one) but I will be let down if Osborne ends up being the top man. Jed Mercurio has constantly steered away from the obvious, and if it’s revealed that the head of the police force is the OCG’s top man, then it’s a let down.

However, a full circle ending could be gratifying and the closure we so desperately want.

Is Marcus Thurwell calling the shots?

It’s a stretch, but believable knowing the scriptwriting prowess of Mercurio. Seen dead with his wife in his Spanish home at the end of the penultimate episode, fans are speculating that the retired police officer faked his own death - and some even think he is Spanish officer who confirmed the death, because of those distinctive eyebrows

Suspicious eyebrows aside, Thurwell does have a corrupt past, and was Senior Investigating Officer on the death of Lawrence Christopher, and had links to the Sands View Boys Home sexual abuse scandal.

Communications from the computer have been linked to Spain, and there is a growing theory that Jo Davidson grew up thinking Thurwell was her real father. This could prove to be correct after the mind-boggling revelations of homozgyosity within Davidson and Tommy Hunter’s family.

Iain’s opinion:

He is currently my firm favourite for the fourth man. He has been calling the shots from his home in Spain, and paying for his lavish retirement lifestyle thanks to money earned helping the OCG.

Although it doesn’t seem plausible that ‘every investigation has led to this’ because Thurwell is such a new entry into the show. But it also seems illogical to bring in an actor of James Nesbitt’s calibre and kill him off after 90 seconds of screen time.

Does H stand for Headquarters?

Have we been steered in the wrong direction believing ‘H’ is a person?

Due to the sheer amount of work this fourth person undertakes, the work could be shared by, and let me put on my best Hastings impression here, ‘a clandestine network of high ranking corrupt police officers’.

Deputy Chief Constable Andre Wise has been continually taking orders from the top, steering Hastings into an early retirement, and going along with the reduction of anti-corruption.

Police Commissioner Rohan Sinwhani showed shades of good after deciding to resign. But what did he witness whilst he was in the belly of the beast, and could it be enough to take down the whole Central Office?

Iain’s opinion:

This seems the most logical of explanations behind the letter ‘H’. The show has done a great job at making us believe that it's a ‘Fourth Man’, but I feel that the OCG has dirt on all the top officials. At least, enough dirt to keep them in their pocket.

The outside contender?

Much like Hastings in season four and five, the camera has begun to paint Kate Fleming in a shady light.

Viewers saw that Fleming left AC-12 after Hasting’s behaviour, but it was her suspicious behaviour in season six, episode five and six that led viewers to believe she’s working with the OCG. Not to mention her full name is Katherine...and there’s a H in there.

Iain’s opinion:

Unfortunately, I think this is step too far. Line of Duty has been brilliant at increasing our suspicions, but we have also forgotten that it's okay for the ‘good guys’ to have flaws. Steve’s ‘mate’ Kate is too good at catching bent coppers to be one herself.

A theory too good to be true?

Steve Arnott has mostly run scot-free from allegations of corruption. But what if the protagonist of the show is really the villain behind it all?

The evidence is thin, but fun to consider.

Could Arnott have been putting on a London accent, and is really part of the Glasgow crime family? Seems far fetched, until you realise Martin Compston is the only actor not using his native accent.

Iain’s opinion:

Through all the morse code pen tapping and James Nesbitt eyebrows, this is by far my favourite theory. Purely because I want to witness the Scooby Doo ending which sees Steve Arnott turn to the camera and reveal his thick Scottish accent.