Robert Hanssen, an FBI special agent, was as successful a spy as America ever had. The trouble was that he did his spying for the Soviet Union. He was only brought to book in 2001, after 500 operatives had spent months stealthily investigating him.

Hanssen had spilt secrets for some 22 years during the Cold War and was clearly more intelligent than most of those who suspected him. He was certainly a great deal more experienced than Eric O'Neill, the 26-year-old special surveillance agent who was detailed to work as his assistant and, if possible, to betray him. He was also, amazingly enough, a devout Catholic and a member of Opus Dei.

Such a story would seem to lend itself to a thrilling movie. But Billy Ray, who both wrote and directed, has made a film that eschews an explosive approach and quietly explores what made Hanssen tick and how a young assistant, also a Catholic, gradually wormed his way into the superspy's confidence.

This is an intelligent effort, illuminated by a first-class performance from Chris Cooper as Hanssen and a pretty good one from Ryan Phillippe as O'Neill. Cooper makes Hanssen a testy perfectionist, constantly complaining that the FBI isn't as up-to-date as it might be and whipping his young assistant into shape like an impatient schoolmaster.

There's also a weariness about his lined face that suggests his life has been a constant struggle with his religious beliefs and what he does. In fact, we never know quite why he did what he did - except that it probably wasn't for money. Perhaps, the film and the actor gingerly suggest, he simply wanted to test himself to the utmost.

Breach is slow, but never ponderous, and not at all like your average spy thriller. It has less obvious thrills and more depth. And it has Cooper, a former Academy winner for his supporting performance in Adaptation, at his best.