Updated on 28 April: For a very long time, Line Of Duty fans everywhere have been plagued by the same seemingly unanswerable question: who is ‘H’?

Now, we know that there are at least four bent coppers in total, because Matthew ‘Dot’ Cottan (Craig Parkinson) tapped it out in his dying moments – and we know the names of three of them; assistant chief constable Derek Hilton, legal counsel Gill Biggeloe and Cottan himself.

So, who’s the fourth? Over the past few weeks, we’ve seen fan theories crop up about Jo Davidson (Kelly Macdonald), Ted Hastings (Adrian Dunbar), Steve Arnott (Martin Compston), Patricia Carmichael (Anna Maxwell Martin), and even our beloved MVP Chloe Bishop (Shalom Brune-Franklin).

But, ahead of the season six finale, there’s a new frontrunner in the ‘H’ race: the heroic, impossibly noble, and seemingly by-the-book Kate Fleming (Vicky McClure). 

And the explosive trailer for this Sunday’s big concluding episode has only added fuel to the fire.

Why? Well, check it out for yourself below:

In the trailer, we see Jo Davidson – who seemingly redeemed herself last week when she took the fall for Ryan Pilkington’s murder – being transported during her arrest, with Kate, Steve and Ted all on hand to escort her. 

As always seems to be the case with police convoys, though, it looks as though things go quickly south, with Kate heard shouting those fateful words of, “Armed police!” 

All of this is intriguing enough, but things get really juicy when Ted is heard saying: “All of these suspicious deaths were orchestrated by one officer in particular. H, the Fourth Man.” 

Cue the AC-12 team poring over transcripts with the misspelled word ‘definately’ highlighted – a tell-tale error made by both Ted and the OCG member that ordered Jo to have Kate killed. So surely… well, so surely Kate’s off the hook, right?

Wrong.

As eagle-eyed fans will no doubt have noticed, the trailer runs through the main ‘H’ suspects in quick succession, including Ted, Carmichael, Philip Osbourne (Owen Teale), and Marcus Thurwell (James Nesbitt). 

However, and it’s a big however, the line-up ends with Kate! 

“For me, H is Kate,” tweeted one fan, adding that she is “the only one that never gets questioned.”

“I’m also starting to wonder if Kate is H!” tweeted another avid viewer.

One more noted: “Watching Line Of Duty back from the start, and Kate is H!”

And still one more insisted: “Argh, Kate’s H! My tin foil hat is ON!!!”

Hmm. It’s certainly a compelling theory – and it would be just like showrunner Jed Mercurio to throw in the mother of all twists in a bid to pull the rug out from underneath viewers’ feet. But what evidence is there, really, to paint Kate as the show’s Big Bad?

Here’s what we have so far.

Kate’s face is on AC-12’s official board of suspects

An Easter egg, a red herring, a reminder that she’s working undercover, or a big fat hint that she’s H?

Your guess is as good as outs.

Kate’s decision to flee the scene in episode six

Look, we know now that Kate shot Ryan twice in the chest, killing him. But, while she was licensed to carry a firearm by AC-12, and it was clear she shot in self-defence, she still fled the scene with Jo.

Why?

And how about Kate letting Jo take the fall for her?

Not only did Kate refuse to surrender to AC-12, and not only did she have to be talked out of dropping her weapon, but she also let Jo take the fall for shooting Ryan. And she did so despite the overwhelming evidence proving that Kate pulled the trigger.

Again, why? It makes no sense!

Carmichael tapping out ‘H’ in Morse code

Look, we all know what we saw and heard (no matter how much Carmichael actor Martin might try to pass it off as a meaningless act of boredom); everyone’s least favourite copper tapped out ‘H’ in Morse code during her interview with Jo. The very same signal, in fact, that Cottan typed out on Kate’s hand as he died.

Some have taken this to suggest that Carmichael is ‘H’. Others, though, have indicated that it points to Kate, as she is the key link between the two tap-worthy scenes; she was there at the moment Cottan breathed his last, and she was the subject of Carmichael’s line of questioning at the time of her own pen tapping moment.

Hmm.

The graffiti in the underpass

As reported on 20 April: In the fourth episode of season six, Ted and Kate met up in a typically graffiti-laden underpass to discuss much-maligned character (and seriously bent copper) Ryan Pilkington.

Ted, naturally, was keen to bring Ryan in for questioning in a bid to protect Kate. However, she insisted that she’d rather “keep observing him.”

“But if he’s who we suspect he is, he could be a really nasty piece of work,” Ted fired back. “I’m only thinking of your safety, Kate.”

“Thank you sir, but if he’s been groomed by the OCG and embedded on the force, he’s linked to the institutional corruption we’ve been investigating,” she replied. “We bring him in, we lose that lead.”

Of course, we all know that Ted’s gut feeling turned out to be bang on the money; the last we saw of Ryan, he had a gun pointed squarely at Kate’s head. But viewers have since picked up on another key detail in the scene…

That’s right; as Ted and Kate talked, the camera angle framed a large graffiti H between them both – prompting many to assume that one of them is, in fact, the mysterious H we’ve long been looking for.

The clue’s in the name

This one’s a little silly, but here goes it; apparently, Kate has been flaunting her status as ‘H’ before us all along.

“Kate is registered as Katherine in the system,” one fan of Line Of Duty recently tweeted.

“Katherine has H in it. She’s the last one. I’m convinced.”

Kate set up the dodgy meeting with Jo

Some viewers have suggested that Kate, after her less-than-positive meeting with Jo (you know, the one where Jo suggested that she ask to be transferred? That one!), jumped aboard her laptop as H to find out what Jo really thought about her… and how far she’d go to take her out.

Think about it; why else would Kate, an experienced AC-12 officer, agree to meet Jo in such a remote and obviously dangerous location? And why did she have her gun with her?

Kate is almost always working undercover within the team of the accused

Think about it; in series one she joined DCI Anthony Gate’s TO-20 unit, then was placed in Lindsay Denton’s Missing Persons Unit in series two

Series three saw Kate placed undercover in Sergeant Danny Waldron’s firearms squad, series four had her join DCI Roz Huntley’s murder investigation unit, and this season she’s working at The Hill with an obviously corrupt Jo Davidson.

Hmm.

The Freemasons link

Remember when everyone decided Ted was H because of his ties to the Freemasons? Yeah, well when Kate later went undercover to investigate a fellow AC-12 officer, she sought the help of the head of another AC branch; SI Madeline Summers.

Cue Summers suggesting that Kate has come to her because a) she is a woman and b) she therefore cannot be part of the Freemasons.

Some viewers have pointed out that this seems… well, it seems more than a little odd. Why, after all, would Summers have jumped to that conclusion without being specifically prompted?

‘Definately’

Look, we know that H has problems spelling the word ‘definitely’ – and we know that, as a result, people have leapt to the conclusion that H and Ted are one and the same person. 

Others, though, have suggested that H is, in fact, dropping the mistake into their correspondence on purpose because they don’t want anyone to figure out it’s them. And, if they were close enough to Ted to know his spelling tics, then they may have purposefully used this one to link it back to him.

Who’s close enough to Ted to know about the ‘definately’ situation? Steve, obviously – and Kate. Eep.

Kate’s spot on the Line Of Duty poster

One fan on Reddit has determined that Kate is ‘H’ because of where she is positioned in the promotional posters of the series.

“OK, hear me out on this one,” the conspiracy theorist wrote. “Whenever you are looking at a Line Of Duty poster or teaser who is in the middle? Kate, despite the fact that Steve and Hastings would have had more screen time.

“Take season two, for example; that series had a lot of basis around Steve and Denton, yet Kate is still in the middle (of the promotion picture).

“My theory on this is that Kate’s position in the posters is saying that she is basically in all of this.”

Kate’s lack of loyalty to her former team

Many have underlined the fact that Kate only began helping AC-12 after her former teammates learned that someone was corrupting murder investigations at The Hill.

Covering her tracks, maybe?

Steve’s big hint in the trailer

The series six trailer saw new AC-12 addition Chloe ask Steve if he still believed there was a fourth man out there.

His response – “Man, or woman.” – was followed by an incredibly lingering shot of Kate looking… well, looking rather suspicious, to be honest.

… but is H necessarily all bad?

Look, we know what the OCG thinks about H – but what if, in reality, H is a false identity set up by Kate (and possibly Ted) to help steer those actually corrupt officers into their carefully laid traps?

Food for thought, eh?

Images: BBC