We’re all being watched. All the time.

That’s a key message of “Closed Circuit,” an entertaining and well-crafted if not overly heart-stopping British conspiracy thriller starring Eric Bana and Rebecca Hall. Security cameras are everywhere, giving us birds-eye glimpses of each character, and reminding us that we, too, are never really alone.

Such a concept is hardly shocking in 2013. After all, we write an email, and soon an ad pops up telling us where to buy that thing we sort of mentioned. And of course we’ve learned in recent months not only of secret government surveillance but even the “Boyfriend Tracker” app for our phones. Perhaps we really do live in a post-privacy era.

But if it’s not a shocking concept, the makers of “Closed Circuit,” an intelligent film directed by John Crowley, have certainly shown how creepy it can be. In the London we see here — one of the most watched places in the world, we learn, in terms of security cameras — you never know who’s around the corner, or who’s been in your apartment, leaving a book slightly askew on your shelf. You don’t know who that cab driver or dinner-party companion truly is. You don’t even know which side your closest colleagues are on.

At least, such is life for Martin Rose (Bana) and Claudia Simmons-Howe (Hall), two lawyers who become ensnared in the legal case surrounding a horrific terror attack, the bombing of a bustling London food market.

As the film begins, we’re staring, fittingly, at footage from security cameras — eventually 15 of them. Each captures a snippet of life on a busy November morning. In one frame, a truck shows up where it’s not supposed to. In an instant, 120 people are dead.

Rowing peacefully on the Thames, Martin gets a call. The lawyer defending the lone surviving terror suspect has committed suicide. Work pressure and all that. Martin’s been tapped to replace him.

As for Claudia, she’s the Special Advocate, an additional defense lawyer designated by British law to examine secret evidence to be presented in “closed session,” away from the public and the press. Even Martin cannot see this evidence.

And he’s not allowed to communicate with Claudia. This is easy at first, since the two happen to be estranged lovers. They’re ambitious enough not to reveal their past romantic entanglement and thus get removed from the case. But if they’re found out, it could end their careers.

And nothing goes according to plan, of course. As the two are drawn together by circumstance as well as their obvious mutual attraction — this is a movie, remember, and lawyers are extremely attractive in movies, even in those odd British wigs — they find themselves having to meet secretly, blatantly defying their superiors.

A smart script by Steven Knight keeps the action humming along smoothly and concisely — if sometimes, it must be said, a bit illogically.

And the two main actors are a pleasure to watch. Bana seethes with frustration and encroaching fear, and looks wonderful doing it. As for Hall, this terrific actress brings the film much of its humanity, striking that difficult balance of competence and determination tempered by a growing recognition of her frailty.

A top-notch supporting cast features the always excellent Ciaran Hinds as Martin’s close colleague, Denis Moschitto as the frightened defendant, Julia Stiles as an American journalist who’s perhaps digging too deep, and, finally, the wonderful Jim Broadbent as the Attorney General — Martin’s boss. You’ve seen Broadbent as Denis Thatcher and as Bridget Jones’ dad; now watch him play an oily official whose cordial smile seems pasted on his face. Never has an invitation to breakfast from the boss sounded quite so unappealing.

“Closed Circuit,” a Focus Features release, is rated R by the Motion Picture Association of America for “language and brief violence.” Running time: 96 minutes. Three stars out of four.