You might expect a bit more caution from a mediocre star making his directorial debut, as Ben Affleck does with “Gone Baby Gone.” First he adapts the Boston-based work of a novelist (Dennis Lehane) last taken on by Clint Eastwood, in the weighty, superlative tragedy “Mystic River.” Then he casts his little brother, Casey, in the lead. And finally, flying in the face of all conventional wisdom, he goes and directs a movie that’s actually not that bad.

It’s fair to put Mr. Affleck front and center in talking about the movie, because “Gone Baby Gone” sometimes feels like a personal project by the Cambridge-raised actor to photograph working-class Boston, uncut and uncensored. The dead-end Dorchester of Mr. Lehane’s thriller galvanizes the film with every drab three-decker and every acne-graced bit character who opens her foul mouth to spray unprintables with a hair-trigger ferocity that makes you glad to be alive.

Fidelity to faces and places is Mr. Affleck’s first good move. The second is to preserve the conflicted moral force of Mr. Lehane’s book, which he hacked down with co-screenwriter Aaron Stockard. “Gone Baby Gone” is uneven, sometimes awkward, but its elemental themes of divided loyalty and close-held principles ultimately resonate thanks to the movie’s sturdy roots in the human qualities that the voice-over calls “the things we can’t choose.”

Casey Affleck plays Patrick Kenzie, a barely credible private investigator whose ace in the hole is his local connections. Working from his spare apartment with his partner and companion, Angie (Michelle Monaghan), he answers the door early one morning to find a distraught woman (Amy Madigan): Her niece has been kidnapped, and her train wreck of a younger sister Helene (Amy Ryan) is too far gone to care. Against Angie’s better judgment, she and Patrick are drawn into the case, and a grave police captain (Morgan Freeman) reluctantly allows them to join a detective (Ed Harris) in tracking down the 4-year-old.

Patrick (and Angie, though she is perilously pared down from the book) amble down various treacherous avenues in their search. Helene’s cocaine habit introduces the specter of a riddling drug dealer who may be involved; the suspicion of a notorious pedophile necessitates a trip to a run-down house out of a horror film. The investigation in the movie streamlines the original plotting and mainly serves as a backdrop to Patrick’s wary, gradually strengthening sense of the right and true course of action when restoring an endangered girl to a criminally negligent mother.

Casey Affleck fits his role like a glove, looking too young, even callow, in a story in which his character grows into a tough moral decision. When Patrick is sussing out leads at the requisite threatening bar (shot on location in South Boston), the claustrophobic menace from the smattering of regulars feels realer for the slight actor than most other actors in most other films. Even churning through the bumpy dialogue, Mr. Affleck gives the sense he would get mauled without his gun, and, more generally, that he might make the wrong decision upon finding the lost child.

Central to the film’s effect, and a well-timed shift, is its “rebooting” halfway through, when Patrick’s understanding of the case seems to hit a snag. The kidnapping crime is deepened by complications that are best left unrevealed but partly related to the tragic family history of the police captain (played by Mr. Freeman whirring into stern-sage autopilot). Along the way, Patrick earns the trust of the prickly Helene (who, in a perfect touch, snipes that the pretty, reserved Angie is still “stuck-up” from high school) and of Mr. Harris’s hard-headed, corner-cutting cop.

The maturity of “Gone Baby Gone” lies in not letting Patrick off the hook at the moment when most movies would anxiously reassure the audience of its foregone conclusions. And that’s a realism that completes the lingering location shots, and the strength that Ben Affleck will have to build on if he sets his next movie anywhere outside the lived-in familiarity of “Gone Baby Gone.” Though nowhere near the Sophoclean intensity or richly developed ensemble of “Mystic River,” Mr. Affleck and his cast chuff along toward a simpler but quite respectable payoff.