Line of Duty star Adrian Dunbar has teased the mysterious H will be unmasked on Sunday night's episode, with a cryptic post on Instagram.
Adrian posted a snap from behind the scenes with co stars Martin Compston and Vicky McClure, who play DS Steve Arnott and DI Kate Fleming.
The actor, who plays Superintendent Ted Hastings, wrote: "Today’s the day fellas. #LineOfDuty... #TedHastings ".
Fans have been left worried the sixth series will end up being the last one, as there is no indication yet that creator Jed Mercurio is making another series - while others are more nervous about who H will be.
One of Adrian's followers replied: "Don't go guys !! hope this isn't the last ever !we won't have anything to get out of bed for if you do".
"If Ted end up being H we will revolt," joked another.
"Oh I can’t cope! Mother of God," quipped a third.
Martin recently revealed the cast don't even know if the show will return, as he explained: "The genuine answer is we don't know, but that's nothing different.
"Every series, Jed takes a couple of months off after it and there's stuff that's way above my pay grade in terms of the data and analytics that comes in.
"There's all this stuff now with 28-day figures because you've got streaming and so on. He likes to sit down in the cold light of day, take the emotion out of it and look at it then.
"I will say for sure that we won't come back just for the sake of it. We'll only come back if he believes there's a story to be told and there's more legs in it," he added, on the BBC's The Afternoon Show podcast.
"There's also a question now of legacy. You know, if it does wrap up, great, do you want to go out with a bang?
"But all that stuff will come into play and I'd say in a couple of months time we'll take a decision."
Meanwhile, Vicky McClure, who plays DI Kate Fleming, posted a final-sounding Instagram post thanking her cast and crew for a wonderful ten years of the programme.
"HAPPY LINE OF DUTY DAY!!!" she wrote.
"Please excuse the slightly over emotional post but it’s been quite a surreal time! The audience support has been unreal, the creativity of merchandise, the artwork, ground breaking viewing figures and the hype!!
"It’s not lost on me how lucky I am to be a part of Line of Duty, not just because of the success of the show but the life long friendships we’ve all gained."
* Line of Duty is available to watch on BBC iPlayer
