Meet the cast:

Detective Chief Inspector Roz Huntley – Thandie Newton

Line of Duty series 4

“I watched the last three seasons of Line of Duty with breathless admiration for its creators, both behind and in front of the camera. I couldn't be more thrilled to be joining them now.”

Detective Sergeant Kate Fleming - Vicky McClure

Line of Duty series 4

“Such exciting news to have Thandie on board. I’m really looking forward to working with her and getting back to Belfast with the team.”

Detective Sergeant Steve Arnott - Martin Compston

Line of Duty

“I’m delighted to be heading back to Belfast with the rest of the AC-12 cast and I’m looking forward to once again trying to do justice to the fantastic scripts Jed Mercurio has written. It's a testament to how well Line of Duty has done last few years that we can attract someone of Thandie Newton’s calibre to the show. It’s great to have her on board.”

Superintendent Ted Hastings - Adrian Dunbar

Line of Duty series 4

“I am thrilled and excited about the new series, as we get the team together for another heart-stopping journey with AC-12. It’s going to be great seeing old friends and making some new ones.”

What is the case?

Line of Duty series 4

Thandie Newton will take on the role of new AC-12 antagonist DCI Roz Huntley, following in the footsteps of Lennie James, Keeley Hawes and Daniel Mays.

Series four begins with Roz and her team in the thick of a career-defining case, Operation Trapdoor.

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.
Determined to uncover the truth, Detective Sergeant Steve Arnott (Martin Compston), Detective Sergeant Kate Fleming (Vicky McClure) and Superintendent Ted Hastings (Adrian Dunbar) must work together to tackle their most devious opponent yet. They are aided by PC Maneet Bindra (Maya Sondhi) and newcomer Detective Constable Jamie Desford (Royce Pierreson).

Writer Jed Mercurio has promised to keep “tormenting the nation” with more “diabolical plot twists”. So nobody is safe and don’t trust anyone.

When does the show return?

The new series starts on Sunday, March 26th at 9pm.

What channel is it on?

The Jed Mercurio drama has been upgraded to BBC One this year, which is a strong show of support from the BBC for the new series.

This third series was the highest-rating BBC Two drama series ever since current measuring system began in 2002, with a series consolidated average of 5.1 million. The exhilarating 90-minute finale consolidated at 5.6 million.

Watch the series 4 trailer


What happened at the end of series 3?

Have you forgotten already? Series three wrapped up a long-running storyline that was threaded neatly through series 1 to 3. The dastardly DI Matthew Cottan, played by the fantastic Craig Parkinson, was finally exposed as the Caddy in an explosive final showdown with Kate Fleming and Steve Arnott.

The Caddy was the corrupt officer who had been co-ordinating dodgy shenanigans for years in the force, dodging AC-12’s interrogations by hiding within their ranks.

Cottan saved Kate’s life in a final act of redemption, jumping in front of bullets fired by one of his criminal allies. Before he died, he was also able to give over evidence that could be used against corrupt ex copper Fairbank (George Costigan).

Keeley Hawes’s enigmatic Lindsay Denton is another character who will be missed in series 4 after she was killed by Cottan.
Neil Morrissey’s slippery Nigel Morton, who has cropped up in every series so far, could make a surprise return, but if he is, it’s currently being kept under wraps.

[Read more: Catch-up on the series 3 Line of Duty finale]
[Read more: Thandie Newton joins BBC drama Line of Duty]