Line of Duty series four: What will AC-12 investigate next?

Line of Duty has been promoted to BBC1 for a fourth series after being crowned BBC2's most successful drama in 15 years.

Creator Jed Mercurio has said he is "looking forward to tormenting the nation with ever more diabolical plot twists" – so what can we expect?

AC-12's finest DS Kate Fleming (Vicky McClure), DS Steve Arnott (Martin Compston) and Superintendent Ted Hastings (Adrian Dunbar) will once again be searching for bent coppers, this time focussing on DCI Roz Huntley, played by Westworld's Thandie Newton. Our Girl actor Royce Pierreson also joins the cast as AC-12's DC Jamie Desford.

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Huntley is leading her team in a "career-defining case" known as Operation Trapdoor, says the BBC.

"Under intense pressure to catch the culprit and prove herself to her superiors, and facing conflict with her colleague, Forensic Coordinator Tim Ifield (Jason Watkins), it's not long before AC-12 decide to probe her handling of the investigation. A mother of two and wife to husband Nick (Lee Ingleby), Roz will do anything to stop her life unravelling. Anything," it adds.

But while the BBC says AC-12 will "tackle their most devious opponent yet", it is open to interpretation if this is Huntley.

A 15-second trailer shows Fleming accusing Huntley of "hiding something", although she insists she is "not corrupt".

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Speaking to Den of Geek, Mercurio said series four would be a "clean slate" in terms of plot, leaving the complex backstories of series three behind

But, he added, there was "always the temptation to throw in a surprise reappearance".