To establish its based-on-a-true-story-ness, “Breach” begins with actual footage from February 2001 of then-Attorney General John Ashcroft announcing that FBI Agent Robert Hanssen has been arrested for selling U.S. secrets to the Russians. We move into the realm of fictionalization then, with actors and scripts re-creating the events that led to Ashcroft’s announcement, but the admirable thing about “Breach” is that it remains realistic and understated even when it is fictional.

This is a movie about spies, yet there is not a single car chase or onscreen murder. The only explosions are of the emotional variety, and even those are few in number. It may be the quietest spy movie ever made.

It’s a riveting one, too, thanks in large part to Chris Cooper, who plays Hanssen as a multi-faceted, enigmatic man. Cooper — an Oscar winner for his supporting role in “Adaptation,” and notable as Kevin Spacey’s homophobic neighbor in “American Beauty” — gives his best performance yet as a man who is prickly at work, stern but loving with his family, devoted to his Catholic faith (he attends mass every day), fervently patriotic … yet who also conducts sleazy affairs on the Internet and trades state secrets to the Russkies. Such contradictions would be unbelievable if they weren’t a) true and b) acted so convincingly by Cooper, whose craggy face and clipped Midwestern twang suggest a no-nonsense drill instructor. I was only in the audience, and I was terrified of having him for a boss.

The unlucky soul actually required to work for him is Eric O’Neill (Ryan Phillippe), an FBI up-and-comer who’s told by his superior, Burroughs (Laura Linney) that while Hanssen thinks O’Neill is just his clerk, his real task is to spy on him. She only indicates the Internet perversion, but O’Neill is smart enough to realize quickly that the Bureau must have more reasons than that for eyeing Hanssen. Why worry so much about the extracurricular activities of a man who will be turning 57 and mandatorily retiring in two months anyway?

The director is Billy Ray (he wrote the screenplay with first-timers Adam Mazer and William Rotko), whose first film was the excellent “Shattered Glass,” the true story of a New Republic reporter who went down in flames when it was discovered he’d fabricated many of his news stories. “Breach” has several obvious parallels to that film, but with a shift in perspective: “Shattered Glass” was told through the eyes of the wrongdoer, while “Breach” is from O’Neill’s perspective, not Hanssen’s. He’s trying to unravel Hanssen’s web of deceit, and we’re along for the ride.

Hanssen is a staunch Catholic and a genuinely good-hearted man. He looks up information on Parkinson’s disease after O’Neill says his mother has it, and when he’s not belittling O’Neill or blustering about inefficiency in the office, he’s trying to bolster O’Neill’s own semi-lapsed Catholic faith. While most religious characters in movies turn out to be insane hypocrites, Hanssen, through Cooper’s honest performance, comes across as a good man who earnestly believes everything he says.

You could want the film to delve deeper into the whys and wherefores of Hanssen’s deeds, and I just noticed that I made the same comment about “Shattered Glass,” too. But “Breach” succeeds without a lot of psychological evaluation of its characters. (It succeeds less when it gets into O’Neill’s obligatory “your job is tearing us apart” arguments with his wife, played by Caroline Dhavernas. The pursuit of Hanssen should be the focus, not O’Neill’s home life.) As a film, it’s smart, it doesn’t pander, and it keeps us intrigued from start to finish.

B+ (1 hr., 50 min.; PG-13, one F-word, a little sexuality.)