Line of Duty, Series Finale, BBC Two

At the end of episode four, we left ferret-faced copper Steve Arnott (Martin Compston) seemingly having his fingers hacked off with a bolt-cutter by a gang of hooded thugs and their poisonous little child-sidekick, Ryan. Boringly, the glum and dislikeable Arnott was rescued in this finale when the supposedly corrupt DCI Gates organised a police rescue, and got away with all his fingers mostly intact.

It seemed to symbolise Line of Duty's annoying habit of setting up ever-murkier scenarios, then wriggling its way out of delivering a real punchline. It really, really wanted us to believe that it was drilling down into a horrifying heart of darkness, but it never managed to take us there, unless you count the way that Gina McKee's body was being trundled around from freezer to freezer in a couple of wheelie-suitcases. And you probably don't (Martin Compston and Vicky McClure, pictured below).

We've sat through five hours of TV in which a variety of police officers - younger, older, male and female, but none of them remotely sympathetic - have tried to put Gates through the wringer and have him convicted of murder and corruption, yet ultimately we had to settle for the fact that he might have been a bit dodgy but not in a really serious way. Which felt kind of feeble, since after shows like The Shield and The Wire, and not forgetting GF Newman's prophetic Law and Order from 1977, we've grown accustomed to the idea that the police force is more likely to be a whole barrelful of rotten apples than just the odd one, and it isn't a question of who's bent so much as the degree of bentness and in which direction. "Every cop is a criminal," as the philosopher Jagger has pointed out. The recent real-life case of PC Harwood did very little to buff up the image of the constabulary.

Still, even snide, malevolent policemen on the payroll of organised criminals probably have a drink and a laugh from time to time, but the cast of Line of Duty were almost unfailingly morose, bitter, angry and disillusioned. Lennie James' performance as Gates has been critically hailed, but his style was severely cramped by the way writer Jed Mercurio had constricted him to a narrow emotional band that oscillated between rage and desperation. Adrian Dunbar's anti-corruption cop, Hastings, made you wish somebody would throw him out of a high window (along with the rest of the cast, if truth be told), though he did manage to smuggle in a few traces of malignant irony.

Actually I did warm - only very slightly, I must stress - to Neil Morrissey's Morton, who eventually displayed some true loyalty to his put-upon boss. Better still, he ungallantly but extremely satisfyingly punched Kate Fleming (Vicky McClure) in the face, for her cynical attempts to schmooze up to Gates in order to destroy his career (Neil Morrissey and Craig Parkinson, pictured above).

But in the end, what did we get? A bunch of captions telling us how the various strands of the story panned out, as if this chunk of hyperactive but self-deluding melodrama had suddenly decided it wanted to be a documentary. It was all doom and despair. The corruption case against Gates was declared not proven and closed... no policemen were prosecuted for the killing of an innocent man in episode one... the smirking Scottish bad guy called Tommy got away with murder... and the viewers got another cleverly-shot cop show with a vacuum where its soul should have been. Bring back the Scandis with their subtitles.