There was a time when American noir films were relegated to the bottom half of double features. Films like "Double Indemnity," "The Maltese Falcon," "Force of Evil," like so many others, were never valued for their insights into an America that had such a dark underside. They were thought of as trivial entertainments, rather than as what they turned out to be, which was classic works of art. I was thinking this as I watched "Gone Baby Gone," Ben Affleck's new film of a Dennis Lehane novel.

The film is a classic noir. A four-year-old child is stolen from her mother while she's out for a half-hour. The cops, headed by Morgan Freeman, are looking for her but her aunt insists on hiring Casey Affleck and his partner Michelle Monaghan, who are local P.I.s and know the neighborhood better than the cops, even though their expertise ends with deadbeat dads and missing wives.

What happens then, to them and about them, as they work their way through a dozen red herrings and some gory, messy crime scenes, is the burden of the film. You could say that everyone lies, and you'd be right. What makes this film stand out for me, even above Lehane's other Boston mystery "Mystic River," is an extraordinary performance by Casey Affleck. He'd already shown us an ability to take the screen away from Brad Pitt in "The Assassination of Jesse James by the Coward Robert Ford," and here, with his strange, almost whiny Boston voice and a manner that takes no prisoners, he is irresistible.

And he's matched by Amy Ryan, who plays the child's mother. An addict, an alcoholic, a foul-mouthed bitch who cares next to nothing about her daughter, Ryan gives one of the bravest performances I can recall, throwing away any hope of enlisting our sympathy in order to be true to the character she's playing.

Affleck directs "Gone Baby Gone" with a very sure hand; he knows Boston, obviously, but he also knows how to stage everything from barroom conversations to confrontations in creepy old houses; and although Ed Harris in a hairpiece as one of the cops is a bit over the top I can forgive the director for that. The only hole in the film is Michelle Monaghan as Affleck's partner; she's barely in the script, and because she has such a conventionally pretty face she's just a distraction from what otherwise would be the tightest, most compelling film in ages.

There is one more, call it moral question that "Gone Baby Gone" raises, and that is, where is a child better off: with a bad, sad mother or someone who is not related but can provide love and affection without end. You can decide for yourself when you see this film.