While all eyes seem to be on newcomer Joanne Davidson (Kelly Macdonald) - Ted Hastings (Adrian Dunbar) is still on the hunt for the mysterious 'H'.
And now, one fan of the BBC series has a very interesting theory that tbh, would make a lot of sense.
Writing on Twitter, they explained: "#LineOfDuty theory, the 4th 'h' isn't a person, it's a place, The Hill. A station where the ocg hold enough sway to constantly rotate bent coppers through the ranks."
And other fans were floored by the theory, with one commenting: "Genius! It would also correlate to Dot's death blinking...H, first letter, second would be a vowel and he didn't make it past G, so second letter isn't A or E, which exonerates Hastings. If he made it to I, then he could have been hinting at Hilton, who recruited him..or The Hill."
Meanwhile another said: "Oh god why has no one thought of this before. Genius. Though have you also seen the H in the tiles on Steph's kitchen wall when Steve goes over? Either an Easter egg or class A trolling from the show..."
Last week, one fan suggested that a clue about H's identity was secretly weaved into the show - in Steph's (Amy De Bhrún) kitchen.
The sneaky clue appeared when Steve Arnott (Martin Compston) confronted Steph about her finances in the wake of her husband John Corbett's (Stephen Graham) death, and eagle-eyed viewers spotted a distinct H pattern on her kitchen tiles.
"'H' finally found in John Corbett's kitchen. Look above the cup rack behind Steve, but Steve totally missed this," joked one Line of Duty fan.
"Anyone else noticed Steph Corbett's kitchen tiles pattern? .... H is everywhere," a second queried.
In other theories, some fans think even Joanne could be the mysterious H.
At the end of episode three, viewers were quick to point out she had been using an identical messaging service to the OCG.
The mystery continues... we are hooked!
Line Of Duty continues Sundays at 9pm on BBC One.
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