As hinted at in the run-up to episode seven, AC-12 finally got to the bottom of the identity of the mysterious Fourth Man, previously known as "H", but the explanation wasn't good enough for many fans.
Jailed detective Ian Buckells was revealed as the criminal mastermind, although Superintendent Ted Hastings (Adrian Dunbar) DI Kate Fleming (Vicky McClure) and DI Steve Arnott (Martin Compston) pointed out that he was more of a "blundering fool" - and Line of Duty fans tended to agree.
Talking about the reveal on Good Morning Britain, Kate Garraway said: "I've got to say - I mean, I haven't seen it yet - but from what's been reported in the papers, it is a little bit disappointing who H turned out to be."
She added: "I don't know, I just expected it to be someone more Machiavellian, moving things around behind the scenes."
Viewers didn't hold back on their disappointment, either, with one tweeting: "that was simply the worst episode and worst ending line of duty have ever and could ever have done #LineOfDuty#LineofDutyFinale"
Another wrote: "I waited 10 years and 6 series for this? Mother of God I’m disappointed #LineOfDuty"
The story went that Buckells had become H by default after more high-profile criminals including Tommy Hunter and DI Dot Cottan had been killed off, and that his role was just to pass on messages to the various splinter OCGs.
Buckells claimed to be cleverer than AC-12 gave him credit for, laughing that he had "made total mugs" out of them, but the last laugh was on him when he became trapped by not being eligible for witness protection if he didn't cooperate with the investigation, but having to stand trial if he admitted to his part in the plot.
Before the episode aired, star Compston admitted to "excitement, anxiety and terror" ahead of the big reveal.
He tweeted: "Done about 40 laps of this flat in the last hour. Excitement, anxiety and terror are a lethal combination. Phone going off for the evening, hopefully we deliver an ending you all deserve."
His co-star McClure also marked the end of the series with a heartfelt tribute to her AC-12 colleagues and show guest stars who this series included Kelly Macdonald.
She thanked fans for their support and said she'd gained "life long friendships".
However, not everyone was disappointed by the ending to season six.
Comedian Marcus Brigstocke tweeted: "I have loved #LineOfDuty . What a vast & brilliant story. I wasn’t wholly satisfied by the end today but so what? I couldn’t have written it. I can’t think what ending would have left me 100% happy. It’s been immense. I hope there’s more. Kudos to all involved."
One fan tweeted: "I think incompetent white men failing upwards as the ultimate baddie is the ending we all needed #LineOfDuty"
Fans are also hopeful that there will be a season seven of Line of Duty, which has not yet been ruled out.
Compston had previously said: "There’s always potential for it to go on. We could always start a new case.
"There’s always corruption out there, as (writer Jed Mercurio) would say, but at the same time where there’s a lot of big questions getting answered in this series, there’s always loose ends.
"He always leaves something dangling for you, whether that’s just a look from Adrian at the end of a series, which might mean something, might not mean something…"
Watch: Martin Compston shares Line of Duty pranks with co-star Vicky McClure