It's the big question the nation wants answered: No, not how Boris Johnson really refurbished his flat (that's second on the list of big questions), but who is "H", or "the fourth man/woman", as H is now known.
On Sunday evening, a large proportion of the nation will be glued to their TV sets, hoping to have the H question and many more loose ends tied up in the final episode of the latest series of Line Of Duty.
In the era of binge-watch TV, heightened during the pandemic, the twists and turns of AC-12's long-running hunt for bent coppers have brought the country together like nothing else. For the last six weeks, fans have dissected and analysed each scene, every little look, in forensic detail, and - Jesus, Mary and Joseph and the wee donkey - found themselves unable to go more than a few hours without quoting their hero, Superintendent Ted Hastings.
Last week's penultimate episode earned an average of 11 million viewers and 51.7% of the night's audience share, the BBC said, making it the most-watched ever episode of the show, which launched in 2012 and has now played out over six series. To put this into context, viewing figures for the Oscars on the same night in the US were just 9.85 million, in a country with a population of 328.2 million, compared with our 66.7 million.
This season's Line Of Duty finale, basically, is the World Cup final of TV drama.
Google searches after each episode go through the roof as fans seek information they may not have spotted, and for those who want more there are podcasts to help fill the gap during the long seven days between instalments. Craig Parkinson (aka bent copper Dot "The Caddy" Cottan of "urgent exit required" fame) hosts Obsessed With Line Of Duty, featuring cast and fan interviews, which at the time of writing is currently sitting at number one in the Apple podcast chart. There's also Shrine Of Duty, which serves up full recaps and, Mother of God, highlights the best "lines of juicy" and fan theories.
As the sixth series of the BBC One show - also starring regulars Vicky McClure and Martin Compston, as well as Dunbar, plus guest star Kelly Macdonald this time round - comes to an end this weekend, a seventh series is still yet to be announced by writer Jed Mercurio. Could this be the last hurrah?
Before we come to that, there are pressing questions from the penultimate episode which need answering.
The new series has seen anti-corruption unit AC-12 take on the investigation into journalist Gail Vella's murder, but Detective Inspector Kate Fleming (McClure) is no longer on the squad and is working on the very murder team under suspicion. Last week's episode saw Acting Detective Superintendent Joanne Davidson (Macdonald) unmasked by AC-12 and some disturbing secrets about her family background revealed.
A trailer released on Monday offered fans a first glimpse of the series finale, featuring Superintendent Hastings saying: "All these suspicious deaths were orchestrated by one officer in particular - H, the fourth man."
The AC-12 team are seen in a shootout before viewers are told: "Every investigation has led to this."
Gulp. As we head into the final episode, here are the big questions that need to be answered. Remember - spoilers!
Who is H/the fourth man?
So many suspects, so little time. Unless, that is, Mercurio is going to leave us dangling once again for the truth to be unveiled in a seventh series.
Chief Constable Philip Osborne is looking more and more like the likely candidate. We knew he was corrupt way back in series one, and he's a senior, powerful figure - "a bare-faced liar, promoted to our highest office", in fact - but he seems too obvious. Ted asked Jo flat out if it was him during her "no comment" interview scene last week, but Mercurio loves to shock, as we know.
Likewise, TV police's most officious operator, Detective Chief Superintendent Patricia Carmichael, is also in the frame. Is she bent, or just a jobsworth? I'd be inclined to go for the latter.
There's also Marcus Thurlwell, who seemingly wins the prize for the shortest guest star appearance in the show's history, even beating Danny Mays's one episode role in season three (plus a few photo appearances). Actor James Nesbitt, who was introduced as Thurlwell via a police database picture before quickly being killed off, hasn't had to do anything for the role so far, other than send in a photo of himself looking shady on the phone. But is he really dead?
Fan sleuths are also pointing the finger at some more unlikely candidates. Why, for instance, has John Corbett's widow, Steph, and her big wad of £50 notes been brought back for this series? And are newcomers DS Chris Lomax and DC Chloe Bishop really conducting themselves to the letter of the law?
And then there's the original AC-12 three themselves: Ted, Kate and Steve. Could Mercurio wind this up by breaking all our hearts and unmasking one of the trio as the criminal mastermind behind it all?
Is this the end for Ted?
"Sometimes you don't lose, son. You just run out of time." These words from Ted cut deep in the penultimate episode of this series, as Detective Chief Superintendent Patricia Carmichael muscled in and Steve was told the gaffer was being forced to retire.
For years now Ted has been hellbent on unmasking H, and it feels like this series has been building up to that moment. However, with so much tension building around his lack of time, there's a horrible sense that this reveal could be at Ted's expense.
If that's the case, Mercurio, we will most certainly not be sucking on diesel.
What will happen to AC-12?
Mercurio is known for drawing on real-life cases for his storylines - most recently the murder of Stephen Lawrence and the unlawful killing of Christopher Alder - so it's no surprise that police budget cuts have also made an appearance.
Anti-corruption teams are being cut by 90% - 90%! Bent coppers will be running rife! - and AC-3, AC-9 and AC-12 are merging.
At the start of this series we learned Kate had left the team. Fans have been hoping for an emotional return, but with no cash in the pot this is looking unlikely.
What else does Jo know, and who did she think was her real dad?
In what was a tremendous performance by Macdonald, Jo Davidson was subjected to one of Line Of Duty's famous lengthy interrogation scenes in episode six. How Mercurio makes so many minutes of "no comment" so intense and exciting is a thing of wonder, but Jo did give away a few bits of info during her time in questioning - including that she had no idea that her uncle Tommy Hunter was her real dad.
Yes, that's dad and uncle, with Line Of Duty credited as the sole reason 11 million people now know what homozygosity means. Her real dad, Jo had told Kate she had always been led to believe, was a bent copper. Is this significant? Could he be H? If not, does she know H's identity? It seems so.
Why did Kate do a runner?
After shooting dead OCG mole PC Ryan Pilkington, Kate and Jo inexplicably fled the scene. As Jo later explained in her AC-12 interview, this was a lawful killing by a serving police officer, as Kate would surely have known, so why the Thelma and Louise?
Kate was authorised to carry the gun and the killing was clearly in self-defence. Plus, Pilkington's previous criminal escapades were all on police records. He was bang to rights! And after finding out Jo had lured her to almost being murdered, why did Kate trust Jo at all? All very strange. A special mention for Kate's slick driving and handbrake turn, though. This woman knows how to make a getaway.
What's under the OCG workshop floor?
"Let's call out the GPR and find out what's under that floor," ordered Kate at the OCG workshop. GPR, for those not familiar with Line Of Duty speak, means ground-penetrating radar.
So what's under there? More £50 notes? Dead bodies? Or bits of dead bodies? Or maybe a big list of all the bent coppers operating in Central Police? Whatever it is, it could be the clue to discovering H.
Is Steve going to be suspended?
He's been popping painkillers and avoiding occupational health meetings all series, and now it looks like he could be in trouble.
But hunting bent coppers is what Steve lives for! Apart from the occasional romantic dalliances with witnesses and suspects and widows of dead undercover officers, the job is all he has! Don't take it away, Mercurio!
Finally, has there ever been a TV character as smug as Detective Chief Superintendent Patricia Carmichael?
The Line Of Duty season six finale airs on BBC1 at 9pm on Sunday