The film opens with a portent of the Sept. 11 attacks: Seven months earlier, then‑Attorney General John Ashcroft announces the arrest of Hanssen with the warning that the United States is an “international target in a dangerous world.”

The action then rolls back two months, as novice FBI surveillance man Eric O’Neill (Ryan Phillippe) gets a surprising promotion to aide for veteran agent Hanssen, who’s been put in charge of a revamp of the bureau’s data security.

Initially, the job seems to O’Neill to be a pesky internal‑affairs gig. He’s been assigned to report to Kate Burroughs (Laura Linney), the agent overseeing an in‑house investigation of Hanssen’s supposed sexual deviancy.

Hanssen’s a hardnose who initially rebuffs O’Neill’s friendly introduction the first day on the job (“My name is Eric,” O’Neill says; “No,” Hanssen barks. “Your name is clerk.”).

Respect and affection quickly take root as devout Roman Catholic Hanssen bonds with the Jesuit‑educated O’Neill and finds himself impressed by an information‑technology report his young apprentice authored.

O’Neill soon learns he’s been drafted into a probe to bring down a colossal traitor, with dozens of agents assigned and the FBI director himself running the Hanssen case.

A subtle cat‑and‑mouse game plays out as Hanssen wavers between paternal fondness for O’Neill and suspicion that his aide is a mole.

The duplicity strains O’Neill’s home life. He’s unable to reveal the true nature of his job to his bright‑eyed, trusting East German wife (Caroline Dhavernas), who is thrust into strange social engagements with Hanssen and his own fiercely religious spouse (Kathleen Quinlan).

“Breach” is a spy story that plays out with cunning moderation and authenticity. No explosions, no guns blazing, except on a pistol range and in a brief flashback.

Yet the film is completely absorbing, a far more taut and cloistered cousin of last fall’s excessively long CIA saga “The Good Shepherd.”

Director Ray skillfully weaves in understated humor (the installation of the Bush administration is marked by the matter‑of‑fact swapping of photos of President Bill Clinton and Attorney General Janet Reno for pictures of President George W. Bush and Ashcroft on a corridor wall).

An Academy Award winner for his supporting role in “Adaptation,” Cooper rarely gets the chance to take the lead. He does so in “Breach” with magnificent flair, presenting a cold, complicated character who’s conflicted by compassion, arrogance, petty desires and phony patriotism.

Phillippe puts in a spirited turn as the eager young G‑men torn between ambition and doubt. Playing an uncharacteristically businesslike role, Linney nicely balances an air of bureaucratic brusqueness with wispy uncertainty over the personal cost of her devotion to the bureau.

Dhavernas, the star of TV’s short‑lived “Wonderfalls,” is a bright new face on the big screen, projecting sweetness and strength as a woman unwilling to accept the toll FBI life places on her marriage.

Dennis Haysbert and Gary Cole offer sturdy support in fleeting roles as agents involved in the pursuit of Hanssen.

“Breach” provides virtually no insight into why Hanssen sold out his country and his comrades. The filmmakers actually account for the movie’s lack of insight in a wonderful musing by Cooper’s Hanssen, where he ponders the motive of another notorious spy, speculating it could have been ego or an urge to expose his country’s lax security.

“The why doesn’t mean a thing, does it?” Hanssen says.

In a movie as engaging as “Breach,” the who, what, where and how more than make up for the absence of why.

“Breach,” a Universal release, is rated PG‑13 for violence, sexual content and language. Running time: 110 minutes. Three stars out of four.