Line of Duty: Episode 1 of series 6 explodes from the traps

Kelly Macdonald’s new character is plunged straight into action in Line of Duty’s sixth series

“Back to basics” is the promise as Line of Duty (BBC One, 9pm) returns for a long-anticipated sixth series. That’s a relief to hear as Jed Mercurio’s crooked copper caper undoubtedly requires a reset. Series five ran aground on the show-runner’s obsession with bombastic plotting and twists that paid off short-term only to make life more convoluted down the track.

The good news is that the latest Line of Duty indeed recaptures the zing of its beloved early seasons. The first of six episodes explodes from the traps, with armed police units and law enforcement lingo flying back and forth in thrilling fashion.

The performances are similarly as manic as we have come to expect. That's with the exception, of course, of the dependably understated Adrian Dunbar as police anti-corruption big cheese Ted Hastings. All hail the Gandalf of overly complex British police procedurals, who utters line such as "hold your whisht" as if delivering an arcane incantation.

As is the tradition with Line of Duty, the storyline is built around a new character played by a big name guest-star. In the footsteps of Thandie Newton, Stephen Graham and Lennie James, this year's marquee signing is Kelly Macdonald, predictably reliable as a high-ranking detective up to no good.

She is plunged straight into the action. Detective Chief Inspector Joanne Davidson is investigating the murder of prominent journalist Gail Vella (Andi Osho). However, when a tip-off leads her to the door of a potential suspect, she seemingly derails the operation by directing her team to instead intervene in an armed robbery. That raid has apparently been staged. Meanwhile, a paperwork "error" causes the police surveying the suspect's apartment to temporarily stand down. The game is riotously afoot.

One component of Line of Duty that tends to be overlooked is the fact it is filmed in Belfast. Mercurio is famously vague as to where in Britain the show is set. And of course it isn't actually shot in Britain at all. The "inbetweeness" of Belfast – a bit British, a bit Irish – brings with it a certain uncanniness. We're not in Kansas, nor even Kent or Cornwall.

Davidson is accompanied on the fateful raid by Vicky McClure's DI Kate Fleming. She ha bailed from the Hastings-led AC-12 anti-corruption unit after season five saw her upstanding boss wrongly accused of nefarious dealings. He's innocent but Fleming has moved on – and seemingly out of the procedural frying pain into the fire.

Still on Team Ted is DS Steve Arnott (Martin Compston). However, he's considering his options, too, and is talking to other police units with a view to jumping ship. That's one of several medium-scale twists in an instalment that generally resists the overkill of season five.

The loudest bombshell goes off at the end as it emerges that Macdonald’s DCI Davidson is in the process of extracting herself from a messy romance with the very copper (Anneika Rose) trying to shop her to AC-12. Wheels within wheels, acronyms within acronyms, Line of Duty is back to its baroque, befuddling best. Fans would have it no other way.