Line of Duty already answered the question 'Who is H?' and it seems like everybody missed it

It'd be easy to just dismiss this as the "monkey with typewriters" effect – with so many fans putting forth so many different possibilities, some of them are bound to be right by the law of averages. But some of the recent detective work carried out by fans of the show, including discovering a number of Line of Duty Easter eggs hidden in the Line of Duty season six trailer, would seem to suggest it's more than that. Line of Duty fans are a canny lot – heck, some might even be in with a shot at a position in AC-12.

It's quite remarkable, then, how quickly a key reveal on the show seems to have been forgotten. In the build-up to the launch of the latest series, both fandom and the media were chiefly pre-occupied with one question, the same one that's been looming over Line of Duty since the end of series three in fact: who is 'H'? "Line of Duty fans uncover 'evidence' to expose H's true identity" one headline read. "Line of Duty female 'H' theory could change everything in season 6" another speculative feature suggested. "DI Kate Fleming 'uncovered as H’ as fans spot major clue in poster" one more story claimed.

It's as if we're all suffering from a very specific bout of collective amnesia. COVID-related delays might mean that it's been two years since the last series, but have we all forgotten that Line of Duty already revealed the true identity of 'H' – a high-ranking officer linked to organised crime – in its fifth series finale? The answer was... no-one. Kind of.

Having spent much of that series teasing the possibility that 'H' could be AC-12's beloved gaffer Ted Hastings (Adrian Dunbar), creator/writer Jed Mercurio then up-ended our expectations entirely with a closing scene which revealed we – and AC-12 – had been attacking the question of the bent copper's identity from entirely the wrong angle. The hunt for 'H' had been kickstarted by the dying declaration of another corrupt officer, Matthew 'Dot' Cottan (Craig Parkinson), who'd appeared to implicate a colleague whose name began with the offending letter. But upon reviewing the footage of Cottan's demise, DS Arnott spotted something...

Having sustained gunshot wounds to the chest, Dot was unable to speak so communicated with DI Kate Fleming (Vicky McClure) by other means, seemingly indicating that his ally's name began with 'H' by blinking when Kate reached that letter of the alphabet. But now, Arnott had spotted that Cottan was also using his hand to send a Morse code signal – "Dot-Dot-Dot-Dot" – and as Steve explained, "The letter 'H' in Morse code is four dots. 'H' is not an initial, it's a clue. [There are] four police staff in league with organised crime."

By the end of the fifth series, three of the offending foursome had already been identified – ACC Derek Hilton (Paul Higgins), Gill Biggeloe (Polly Walker) and of course Cottan himself – leaving one more villain still at large. In some ways, then, this reveal didn't change all that much – AC-12 remain on the hunt for a single corrupt cop in league with the OCG – but what this twist did do was dramatically lengthen the list of potential suspects. Whereas previously 'H' was thought to be an initial and all eyes were on the likes of Hastings, Lester Hargreaves (Tony Pitts) - who was corrupt, but wasn't 'H' - or even series four antagonist Roz Huntley (Thandie Newton), now it could be literally any high-ranking officer within the police.

Why, then, are we still talking about 'H'? Was everyone so fried by Line of Duty's thrilling but emotionally exhausting 90-minute series five finale that they missed this big reveal?

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Of course not. The truth is it's just Habit – 'H' is a quick-and-easy way for fans/the media to refer to the final corrupt officer, even though it's no longer strictly accurate. 'Who is the fourth Dot?' isn't quite as catchy, and is just as confusing in its own way. But as we prepare to embark on another investigation with AC-12, it is worth stressing that the 'H' rule no longer applies and that practically any member of the Line of Duty cast could be the Big Bad in question – even one of our trusted lead trio.

The series six trailer saw new AC-12 addition Chloe Bishop (Shalom Brune-Franklin) ask Arnott if he still believed there was a fourth man out there, his response – "Man, or woman." – being followed by an ominous shot of Kate looking rather suspicious. Previously, justifying how Kate could be 'H' required something of an absurd logical leap. Jed Mercurio recently told Radio Times magazine that one fan had asked him if there was some significance to the way Fleming holds her Handbag, in case this was the vital clue we'd all missed. Now, though, the net has been widened and Kate is as solid a suspect as any.

So what do we need to remember heading into Line of Duty series six? Just this: there is no 'H', but there is one last rotten apple to be identified... and pretty much everyone is a suspect. The next seven weeks are going to be stressful...

If you’re a budding writer or a massive TV fan, you don’t want to miss our exclusive event, Script to Screen with Jed Mercurio. You can find out more information about the event, or purchase your tickets here.

Line of Duty returns on Sunday 21st March at 9pm on BBC One – head to our Drama hub for all the latest news, or visit our TV Guide to find something to watch tonight.