"Closed Circuit" comes at a moment when the U.S. National Security Agency has admitted to extensive spying on civilians and Britain employs half a million closed circuit cameras in London. So not surprisingly a film that posits that governments can spy on whoever they want, whenever they want, ends up being advertised in magazines like The Nation with the tag line "They See Your Every Move."

Still, when you are telling this kind of inevitably contrived story, you need a good script, strong direction and expert actors to really sell it, and that's what "Closed Circuit" — written by Steve Knight, directed by John Crowley and starring Eric Bana and Rebecca Hall with fine supporting work by the likes of Ciaran Hinds, Riz Ahmed, Julia Stiles andJim Broadbent — provides.

Knight, possibly best remembered as the co-creator of "Who Wants to Be a Millionaire," is also the screenwriter behind Steven Frears' excellent "Dirty Pretty Things." His new film starts with a bang — literally — and then joins a potboiler plot with some interesting ruminations on what lengths a state will go to protect its citizens — and its own interests.

"Closed Circuit's" premise is a complex one that carefully unfolds after that initial bang — a massive bomb detonated in London's Borough Market that kills 120 people — is displayed from the simultaneous vantage points of a dozen security cameras.

Cut to several months later, and the run-up to what the British media are calling "the trial of the century, one of the most high-powered criminal cases in British history." Farroukh Erdogan (Denis Moschitto), said by the government to be the only surviving member of the terrorist cell that did the deed, claims he was an innocent bystander who was only renting space to the bad guys.

Because this is a case involving terrorism, a legal wrinkle peculiar to Britain goes into effect that gives Erdogan two attorneys. While one lawyer, barrister Martin Rose (Bana), will be his attorney in open court, he will have another one, Special Advocate Claudia Simmons-Howe (Hall). She is appointed to hear the classified evidence that the government feels needs to be kept secret for reasons of national security.

It is part of the British system that these two lawyers cannot meet, share information or have any sort of a relationship. So the fact that Martin and Claudia had a clandestine affair that broke up his marriage and have decided to keep their involvement a secret from their bosses is a huge risk for both of them.

Not surprisingly, this case gets way more complicated than either Martin or Claudia anticipates, and involves a number of important folk, all ably played: the attorney general who oversees all things legal (Broadbent), the solicitor who got Rose the job (Hinds), an MI5 operative with close ties to the case ("The Reluctant Fundamentalist's" Ahmed), even a feisty investigative reporter for the New York Times (Stiles).

Crowley, best known in Britain for his theater and television work, does a brisk job of keep these multiple trains running on time. Given the subject matter, an especially nice touch is that cinematographer Adriano Goldman made use of small digital cameras to frequently show us a surveillance camera point of view of the action.

It's also a pleasure to see how much energy co-stars Bana and Hall bring to their roles. While Bana is all he should be as a frequently frustrated Type A individual, it is Hall, one of the most gifted and versatile actresses of her generation, who really has her way with the script's tight dialogue.

Trim and effective though "Closed Circuit" mostly is, it does fall prey to excessive contrivance from time to time, as most thrillers do. But the fact that its fictional premise dovetails nicely with what we've come to know is true is enough to hold us in our seats.

kenneth.turan@latimes.com