Line Of Duty, BBC One

Ahead of its return to our screens last Sunday for a fifth series, Line Of Duty was being promoted by the BBC as a show “from the makers of Bodyguard”, by which they meant writer Jed Mercurio and production company World Productions, headed by TV-land legend Tony Garnett. Bodyguard, let me remind you, has only a single series in the can so far, though admittedly it was a belter. So while I suppose it’s only fair the Beeb use it to hit reset on Line Of Duty and try to pull in a new audience, it rather diminishes the more established show’s attributes. Indeed it wasn’t so long ago (series three in fact, the one starring Keeley Hawes) that the nation was as gripped by Line Of Duty as it was by Bodyguard.

That said, Line Of Duty brings with it a feeling of déjà vu. The opening episode kicked off with a police convoy being hijacked, as did series three, and by the end of the episode we were yet again questioning the motives of Superintendent Ted Hastings (Adrian Dunbar), head of AC-12, the police anti-corruption unit at the heart of the drama. We had also had the rug pulled from under us in typical Line Of Duty fashion when it turned out that the UCO (undercover police officer) in the badass OCG (organised crime group) behind the convoy hijack wasn’t who we thought she was. In fact she wasn't a she at all, she was a he who appeared to have gone rogue – John Corbett, played in trademark menacing fashion by the always watchable Stephen Graham. There is a she in the gang, but it now seems that she's a straight-up, honest criminal.

Mercurio calls this “swinging the pendulum”. But the worry is that the pendulum’s swings might become so easy to plot that some maths wonk somewhere will have an algorithm online by the end of episode two telling us who’s good, who’s bad and maybe even why Greenock native Martin Compston plays Detective Sergeant Steve Arnott with a strangled Estuary accent. (That has always irked me, as has the show’s deliberately non-specific location. It’s filmed mostly in Belfast but it isn’t set there. It might seem like a small point but geographical specificity adds value to cop shows – think Shetland, Hinterland, The Bridge, The Tunnel).

Still, everyone loves a good mole-hunt and Mercurio’s pendulum swinging is skilful enough to still wrong-foot us regularly. The larger, over-arching, series-to-series plot about a deep police conspiracy also adds a touch of X-Files paranoia, though let’s not take the comparison too far: I don’t expect we’re likely to see Steve Arnott and his AC-12 colleague Kate Fleming (Vicky McLure) in a will-they/won’t-they relationship any time soon. Then again, this is Line Of Duty. Expect the unexpected.