Over the years, Line of Duty has gained a reputation as one of the most confident, propulsive dramas Britain has ever produced. Every series is drum-tight and packed with gasp-inducingly bravura plot developments. It’s the closest thing we’ve ever had to 24. Line of Duty is a speeding locomotive, albeit one that has to speed by necessity because it falls apart the moment you apply any level of meaningful thought to it. Over the years, it has built a following thanks to its sheer next-day oh-my-god-did-you-SEE-that word of mouth. It was exceptional.

But this year, something went wrong. Series five has ended not with the clench-chested adrenaline rush of previous instalments, but a slightly defeated shrug. And I think I know why.

At the end of the last series, creator Jed Mercurio tore away the curtain to reveal that the whole show had been connected by the same overarching conspiracy all along, run by a corrupt high-ranking officer known only as H. It was an unbelievably bold moment, one that looked set to move the story forwards with a violent thrust the likes of which we’d never seen.

And yet now here we are, with the story stuck in exactly the same place. Take a god’s eye view of Line of Duty and nothing has changed. There’s still a grand conspiracy. H is still at large. He still probably isn’t Ted Hastings. Steve Arnott has still got a sore back. Years from now, when you come to rewatch Line of Duty as a whole, you could skip series five entirely and it wouldn’t make the blindest bit of difference.

Instead, to make up for this loss of momentum, we got to watch Mercurio indulge in his favourite trick; bumping off the best character halfway through. For four episodes, Stephen Graham’s undercover officer John Corbett held the whole shebang together. He was such a presence – angry and conflicted, all veins and cartilage – that the season quickly became his story. So when he died, just as when Keeley Hawes died in Bodyguard, your reaction tended to be disappointment rather than shock. And, as with Bodyguard, the story collapsed without its focal point.

He’s done it again! ... Jed Mercurio is always bumping off his best character halfway through a series. Photograph: Aiden Monaghan/BBC/World Productions

This isn’t the last we’ve seen of Graham, by the way – from what I’ve watched of it, his work in Shane Meadows’ The Virtues is likely to be the television performance of the year – but he deserved better. Especially since, once he left, Line of Duty reverted to the silly old standby of How Bent Is Ted Hastings?

And of course he isn’t bent. He’s Ted Hastings, for crying out loud. He’s the most perfect man who ever lived. He’s the sort of man who gets so embarrassed by watching pornography that he immediately has to destroy his computer the second he’s done with it. Ted Hastings is a paragon of righteousness, and – despite the several thousand shifty-eyed cutaways Line of Duty has treated us to over the last month and a half – nobody ever believed otherwise.

More than anything else, presenting Hastings as a possible criminal overlord was a waste of Adrian Dunbar’s talents. One of the thrills of Line of Duty has always been watching Dunbar’s jazzily offhand delivery undercut the blunt-force trauma of Mercurio’s dialogue. He has always been a humanising component in what might otherwise be an offputtingly sterile production. So to have him reduced to a series of suspicious fake-out reaction shots – to have him become the guest star, essentially – robbed the audience of the one thing that bonded us to the show.

And, increasingly, the decision to tie the story together is also starting to look like a big mistake. Not only did it mean that Mercurio had to dig back all the way to the show’s first interrogation scene to construct a shonky backstory about maternal revenge, but it meant that most of Sunday’s finale was spent explaining it. The fact that Mercurio wrote an almost unbroken hour-long interrogation is deeply impressive in itself, but dedicating so much of it to describing something that happened 30 years ago was a hell of a patience tester. It was all tell and no show, and it overestimated how much anyone cares about the wider conspiracy.

If it keeps up like this, Line of Duty is destined to drown in its own mythology. There will definitely be a sixth season, so it still has a chance to recover. It’s easily achieved, because Mercurio is already the very best at what he does. Deliver us a lean, dynamic series full of punchy interrogations and breathtaking setpieces and all will be forgiven. So long as he doesn’t murder his best character four episodes in, obviously.