Line of Duty series 3 preview: Danny Mays is the perfect choice to play that awful mixture of victim and perpetrator

The role of apparent villain for this upcoming run is filled instead by Daniel Mays who plays Sergeant Danny Waldron, leader of an Armed Response Unit, whose personal troubles seem to put Denton’s in the shade.

Line of Duty has an established pattern of introducing an “antagonist” (the description used by creator Jed Mercurio) who prompts anti corruption unit AC12 to investigate. And in an intriguing twist on what has come before, the apparent culpability of Waldron feels clear from the outset.

The opening scene finds his armed unit chasing a gangster. Waldron runs ahead of this colleagues and something happens – which I will not spoil – which is both highly violent and deeply suspicious. But of course this is Line of Duty and obviously there is much more to this than meets the eye.

Quite what Waldron's motives are is not entirely clear (though they are strongly hinted at) and the first episode is taken up with the start of the investigation and the deep annoyance of Waldron’s colleagues whom he strong-armed (to put it mildly) into lying on his behalf about the incident and wants singing from the same hymn sheet when the investigation starts.

But there is little doubt that he is a compelling character. He seems to be someone who has suffered terribly in his life, but is himself also a ghastly bully. There is pain and brutality etched onto his face and Mays – an actor who has excelled on stage in the plays of Harold Pinter – is the perfect man to play this awful mixture of victim and perpetrator.

It’s an impressive opener to a series which once again explores the fascinating tension between coppers investigating coppers and the unique frisson that goes with that. Mercurio is excellent on the detail of police procedural – we discover exactly how officers are issued with firearms, what they have to sign, and how it all works. Waldron too is someone who knows the system, making him incredibly smart and slippery.

So time will tell if he will see through Vicky McClure’s sharp-eyed DC Kate Fleming who is sent undercover into the unit to try to find out what is going on.

Martin Compston’s ferret-like DS Steve Arnott also yaps away at the investigation. And there is a welcome return for the great Adrian Dunbar as grizzled Ulsterman Superintendent Ted Hastings, the boss with the marriage problems (which Dunbar tells me we will be exploring in more detail).

Craig Parkinson’s corrupt copper Matthew “Dot” Cottan makes an appearance and it is unclear whether he too will come under suspicion; no one except viewers who have seen the first two series are aware that he’s a total and utter wrong ‘un.

The first two series really exposed the rot at the core of the police. But if anything, this first episode suggests that series three may take a softer, dare I say it, more sympathetic line regarding the force.

There are plenty of moments where Hastings emphasises the importance of getting to the root of police corruption. And on the evidence of the first episode there is less of a sense that you will go away from this in as much despair about the integrity of the boys in blue as you may have done in series one and two.

“We are not a police bashing show,” Mercurio said at the Bafta screening of the first episode. “We hope that in watching the series people will understand the kinds of situations that police officers deal with are ones that very few other professionals have to face and that is obviously great for us dramatically.”

Yes, that is all true. But this being Line of Duty, quite where we are going is anyone’s guess. And that is its particular joy and brilliance.

Line of Duty series three begins Thursday 24 March at 9pm on BBC2