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When someone falls from a bridge in front of a train, Mal Craig, SIO with the British Transport Police, is called in. At first classified as non-suspicious, the death soon turns out to be much more complex than Mal's colleagues first thought.

Meanwhile, at home, Mal's struggling to come to terms with the collapse of his marriage and to persuade his estranged wife to allow him access to son, Luke. This becomes all the more difficult when it turns out that Luke's activities are of interest to the investigation.

"Mal's a policeman with the British Transport Police who is called in to investigate what first looks like a suicide of a young girl, and they quickly discover that it is not as straightforward as they first thought," says the versatile actor, recently seen as the interrogator in Mrs Mandela, on ´óÏó´«Ã½ Four, and also in the British film Nowhere Boy.

"The case is taken away from him by the regular police force, but he still has to work on it. He is no longer the leading officer, he is subordinate to Superintendent Jim Carpenter, played by Hugo Speer. Mal finds this shift in power quite difficult," says the star of Blackpool, The Deal, Doctor Who and State Of Play.

"There's also a rookie cop, Laurie Franklin, played by Suranne Jones, who has come in to help them – she is tenacious and doesn't play by his rules. They sort of rub each other up the wrong way and that relationship becomes quite conflicted," continues the RADA-trained actor.

"In his personal life he is going through a divorce. He has a son he adores but finds it very difficult to spend time with him and, when he does have time with him, he is slightly distracted by his job, allowing it to get on top of him.

"What I liked about the character is the fact that it's about a man awaking to his son and really finding the value of family. I think the series is about family and that's what I love about it.

"There is this incident which happens at the beginning of the film which spreads out like a spider's web and touches so many different characters, but the central theme, it seems to me, is all about family and how we deal with family."

With a young family of his own and a number of Hollywood films already under his belt, would David consider upping sticks for the bright lights of Hollywood for long periods of time?

"I have young children who are in school – it'd be hard to go off to America for a year. I'm very busy here in Britain, I'm enjoying the work I'm getting and the challenges. Sometimes it's a case of the grass is greener on the other side but I'm really enjoying the work I'm doing here.

"When I have worked on American projects I've really enjoyed them, I like the way they work – they have a passion for what they do," says the Liverpool-born actor, whose credits also include Derailed, starring Jennifer Anniston; the horror film The Reaping, with Hilary Swank, and Captain Correlli's Mandolin.

"Acting is my first love – I'll always be an actor who directs rather than the other way around. Although I love directing, it's all-consuming – you can't do anything else.

"The other great thing about being an actor is you don't know what you're going to be doing next year. The first half of 2009 I was on-stage, then in the Cairngorms, then Liverpool doing Nowhere Boy and Yorkshire filming Five Days.

"At the same time I was also getting the film I directed, Don't Worry About Me, ready for the London Film Festival. Don't Worry About Me, due to be broadcast on ´óÏó´«Ã½ Two in March, did really well at the Festival. I'm just about to take it on tour around Britain."

Finally, following speculation in the media last year about the replacement for David Tennant, Does David have a hankering to play the famous Doctor?

"I get asked about Doctor Who all the time!" laughs David. "I was really proud to be associated with the show. And also because I know David so well.

"I thought he was a fantastic Doctor, not just in how he played the character but how he took over the whole role of being the Doctor. When I was with him, he treated people wonderfully – people asking for autographs, people bringing their children to the set – I really thought he was wonderful.

"It's a great project to be associated with. I think Matt Smith is a wonderful actor – I'm really looking forward to seeing him. But, you never know, if they go for another incarnation, maybe it'll happen then, but we'll have to wait and see. Never say never."