Line of Duty series four: All you need to know

BBC police drama Line of Duty returns for a fourth series this month, with a gripping new corruption case for AC-12 to investigate.

Who is in it?

This season's questionable cop is Thandie Newton, who is best known for her role in HBO's Westworld, which won her a Golden Globe, and her Bafta-winning performance in Crash.

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Most of the key characters from the previous series are returning, so fans can look forward to seeing their favourite trio of superintendent Ted Hastings (Adrian Dunbar), DS Steve Arnott (Martin Compston) and the newly-promoted DS Kate Fleming (Vicky McClure). The new series also includes the return of PC Maneet Bhandra (Maya Sondhi) and a new AC-12 addition in the form of Royce Pierreson's DC Jamie Desford.

What's the new series about?

As with earlier series, the plot involves AC-12 investigating a case of corruption or unethical police behaviour.

This time, the team is investigating DCI Roz Huntley (Newton), an ambitious senior investigator who seems to have cracked a serial-killer case known as Operation Trapdoor a little too easily.

AC-12 must determine whether Huntley is playing by the rules or whether she is using dubious methods - and possibly even framing a suspect with learning difficulties to claim the credit for cracking the career-boosting case.

What happened in the last series?

Season three of Line of Duty saw AC-12 finally discover which of the team was leaking information, uncovering DI Matthew "Dot" Cottan's role as a double agent in the police anti-corruption unit.

Dot (Craig Parkinson) made a last-minute bid to escape justice, but was shot in the process, although not before confessing to Kate.

What can we expect in the future?

No one is safe on Line of Duty, says Morgan Jeffery in Digital Spy, revealing that one alternative ending to the last series even saw Steve Arnott being killed.

Clearly, though, AC-12 will continue to operate "because the Beeb's already ordered a fifth series", he adds.

Is there a trailer for the new series?

Yes, and you can see it here:

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When and where can I see series four?

Previously a BBC2 drama, Line of Duty has been upgraded to BBC1 following the success of the third season, with the first of its six episodes being broadcast at 9pm on Sunday 26 March.