LINE OF DUTY Jed Mercurio’s gripping police-procedural series is due to return for its sixth season before the end of March with further tales of AC-12’s ongoing struggle against corruption in the UK police force — corruption which may or may not trace back to the officer in charge of the anti-corruption unit. The cast regulars are all returning, while Scottish actor Kelly Macdonald (“Trainspotting,” “Giri/Haji”) is signed up as Detective Chief Inspector Joanne Davidson, whom Mercurio has described as “the most enigmatic adversary AC-12 has ever faced.” As always, details of the storylines have been kept tightly under wraps, but it’s a good bet that, somewhere down the line, there will be one of Mercurio’s trademark breathtaking twists.