I hate it when people describe television as a rollercoaster ride, but there is one sense in which Line of Duty (BBC One, Sundays 9pm) fits the description: your heart is racing before it even begins. Four seasons of this cop drama have trained the body to expect thrills. Each one has been nastier and battier than the last.

Our heroes are the anti-corruption unit AC-12, which exists to police the police. Each of them could do with a firm hand themselves. Superintendent Ted Hastings (Adrian Dunbar) has a broken marriage he can’t let go; Detective Sergeant Kate Fleming (Vicky McClure) has kids she never sees; Detective Sergeant Steve Arnott (Martin Compston) is a 4ft-tall Casanova. And whenever these pen-pushers enter a room, the temperature drops. Can you blame the other rozzers for hating AC-12? Imagine how hard your job would be if tiny Steve Arnott turned up at 5pm every day and went through your briefcase to make sure you weren’t nicking the stationery.

That said, the rotten apples in the fictional world of policing are very rotten indeed. We’re talking pimping, assassinating, thieving and – in this brand new series – possibly stitching up a mentally handicapped man for a crime he never committed. Is the copper under investigation truly bent? If so, why? That’s the question at the heart of every season of Line of Duty – and the writers’ ability to keep us interested in a whodunit – where we pretty much already know who did it – is quite special.

There are flaws. Every season is bookended: a stunning first episode and a nerve-shredding finale with four hours of padding in between. And if the thin blue line is as ludicrously crooked as this show makes out, then we need to think about abolishing the police altogether. But getting the audience to suspend disbelief is the mark of a fine thriller – and this is one of the best. In a few years’ time, we’ll talk about Line of Duty in the same way that we do Prime Suspect. Nowhere near as psychologically deep, perhaps, but capable of evoking an atmosphere of corruption and immorality that sends you to bed with shaking bones. Just like disembarking from a rollercoaster.

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