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AC-12 have found a trail of clues which seemingly leads to the 'fourth man' - a senior police figure leading a series of bent coppers who then allowed the OCG to continue with their underhand dealings.
However, while many of us have our money on the supremely unlikable Patricia Carmichael (brilliantly played by Anna Maxwell Martin) or the allegedly 'dead' Marcus Thurwell (a surprise cameo from James Nesbitt), other Line of Duty fans believe H has been staring us in the face all along.
Yep, some armchair detectives have sleuthed that it is actually DI Kate Fleming (Vicky McClre) is the link between the police and the OCG.
Fans were left bemused on how Sunday night's episode, which picked up following the shoot out between Kate and bent copper Ryan Pilkington (Gregory Piper), saw both Kate and Joanne Davidson (Kelly Macdonald) flee the scene.
Kate had shot Ryan twice in the chest, killing him. While she was licensed to carry a firearm by AC-12, and it was clear she shot in self-defence, it didn't seem to make sense why she would run away with Joanne as it only made her look even more guilty.
Kate raised more eyebrows after she refused to surrender to AC-12, and had to be talked out of dropping her weapon. She then let Joanne take the fall for shooting Ryan, despite overwhelming evidence she committed the fatal action.
The plot thickens if we observe the Line of Duty finale trailer, which sees Ted Hastings (Adrian Dunbar) declare that a number of suspicious deaths were orchestrated by 'H', before a number of suspects flash up on screen.
As well as more obvious shouts like Patricia Carmichael, Chief Constable Osborne and even Ted himself, the final 'suspect' we see is Kate.
And things get even fishier if you look back throughout the series...
In a meeting between Hastings and Kate in a graffiti-strewn underpass, one savvy fan spotted a large H between the two of them. Another clue, perhaps?
Another fan said: "Kate is registered as Katherine in the system. Katherine has H in it. She's the last one. I'm convinced."
Okay, so we're less persuaded by that theory.
But Kate is an experienced undercover officer, having been placed in numerous teams throughout the years to infiltrate and analyse potential corruption.
Was she playing the ultimate double bluff all along by being a member of the OCG placed in AC-12? Mother of God, we just don't know anymore!
Line of Duty concludes Sunday at 9pm on BBC One.
Featured Image Credit: BBC
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