‘Line of Duty’ series 5 finale review: We finally reach the ultimate reveal – but heavens, it takes an age to get there

The fifth series of Jed Mercurio’s police procedural winds its way to an exceedingly talky conclusion

Jed Mercurio loves to lull his audience into a false sense of safety, and then cosh them over the head with a shocking reveal. But in the exceedingly talky, feature-length final episode of Line of Duty’s fifth series, he delivers the blow with a feather duster rather than a truncheon.

Talky, in fact, is an understatement. This is a chin-wagger for the ages, and though the show – which has shattered its previous viewing records this year – wraps up most of its loose ends, it does so with excessive leisureliness.

The ultimate reveal is that AC-12 chief Ted Hastings (Adrian Dunbar) is innocent of the corruption charges against him – specifically the accusation that he is criminal mastermind “H”. Evil lawyer (has there ever been a non-evil one on a police procedural?) Gill Biggeloe (Polly Walker) was orchestrating a massive stitch-up against him the entire time.

But heavens, it takes an age to reach the pay-off. Along the way, we must sit sweating with Hastings as, minute by minute, he is grilled by dead-eyed careerist detective Patricia Carmichael (Anna Maxwell Martin). Hardcore fans may have appreciated the slow-burn pace after an action-heavy series. Anyone rubbing their hands in anticipation of an explosive finale, though, may be underwhelmed by the emphasis on banter over blazing bullets.

With one eye perhaps on viewers who – tut, tut – haven’t been completely paying attention, taking notes etc, Mercurio has Carmichael rehash all the evidence against Hastings. The £50K in the big brown envelope, that laptop wiped at a convenient moment (he claims it contained his porn stash), his murky past serving with the RUC in Northern Ireland and, in particular, his questionable friendship with a widowed IRA informant.

He sighs and sweats. She looks so very cross. Perhaps the intention is to allow two formidable performers go head to head. But there isn’t much antagonistic chemistry. Dunbar and Maxwell Martin seem to be acting past one another.

What momentum the 84-minute episode does possess is courtesy of the ever trustworthy DI Kate Fleming (Vicky McClure) and DS Steve Arnott (Martin Compston). They’ve gone back to Liverpool for a repeat quizzing of the wife of dead undercover copper John Corbett. Hastings stands accused of having Corbett executed by leaking his undercover status – part of his evil machinations as the omnipotent “H”.

Helpfully, Corbett’s wife caves in and furnishes crucial evidence that will clear Hastings’s name. And just in time, too. Back at the interrogation room, DNA data (faked by Biggeloe) is produced that connects Hastings to Corbett, though he insists they’ve never met.

Game, set and match Carmichael – until Fleming and Arnott charge in with a secret recording kept by Corbett (and handed over by his wife). Here, we hear Biggeloe misidentify Hastings as the man who sold out Corbett’s mother – yes, the IRA informant – to paramilitaries back in Belfast in the Eighties. It’s all part of her plot to weaponise Corbett, and use him to take down the most honest cop on the force and a thorn in the flank of organised crime.

Biggeloe, it transpires, went over to the criminal dark side while working as a defence lawyer – a clue she dropped previously. Of course, removing Hastings would also be a boost to senior police management made to look incompetent by his unit’s relentless pursuit of rotten apples. Everyone would be a winner (aside from pesky Hastings and AC-12).

Towards the end, there is finally some action. One of Carmichael’s officers turns out to be an evil secret ninja assassin working for the bad guys, and gorily stabs potential informant Biggeloe through the hand in the lady’s loo.

Yet Biggeloe is ultimately just a minor player, and the search for “H” is confirmed to be ongoing as Fleming and Arnott, reviewing the dying Dot Cottan’s final message from series three, work out that there are actually four evil police overlords pulling the strings.

As he bled out, Cottan spelt “dot, dot, dot, dot” in Morse code. His final utterance of “H” was not an initial but a clue (“H” in Morse code is four dots). “Four dots… four police staff in league with organised crime.”

Three, we are reminded, have been identified in previous series. Who is number four? Hastings looks perturbed. But is that a concerned expression, or the frozen features of – cue potential Mercurio twist – a guilty man?

Line of Duty may have wended its way to a chatty conclusion this year. But with a sixth series already green-lit, it’s clear there is still a whole lot of mystery left to unpick.