It’s fun to mock Ben Affleck. In the past five years, while his old Boston buddy Matt Damon quietly earned cred in the Bourne movies, Affleck has distinguished himself mainly with bad action roles (Daredevil) and a stint as Jennifer Lopez’s arm candy. Sure, he had creative ambitions, but the TV series he produced tanked, and the independent movies he greenlit on “Project Greenlight” mostly sucked.

Now Affleck has directed his very own movie, Gone Baby Gone. And — unfortunately for those who enjoy tracking an Oscar winner’s demise — it’s good. Really good. Affleck made the smart decision to adapt a dark, Boston-set novel by Dennis Lehane, author of Mystic River and a frequent writer for HBO’s gritty series “The Wire.” Hiring actors from “The Wire” for supporting roles was another good move. But Gone Baby Gone really distinguishes itself as the rare movie in which nepotism works. In the lead role of private detective Patrick Kenzie, Affleck cast his little brother Casey — mainly known in the past for having the same lashy good looks as Ben with none of the presence.

In this role, though, the young actor’s air of callowness fits. He’s playing someone who’s in way over his head. When Kenzie and his partner-girlfriend (Michelle Monaghan) get involved with the high-profile investigation of a missing 4-year-old girl, neither has ever worked an abduction case. But little Amanda’s aunt (Amy Madigan) wants to make sure all the bases are covered. So she hires Kenzie, a kid she knows from the neighborhood. Maybe someone who’s spent his life on the mean streets of Boston’s Dorchester will know how to ferret out info the cops can’t.

Sure enough, Affleck’s character uses his charm and old high-school connections to delve into the background of the missing girl’s mom, whose tears the local TV stations are pumping for ratings. When her daughter vanished, she was down the street doing lines of cocaine at a dive bar, and she may have aroused the ire of a local drug lord. As Kenzie feeds information to the police detective assigned to the case (Ed Harris) and his boss (Morgan Freeman), they start to respect the newbie. Meanwhile, though, the chances of Amanda’s survival are decreasing by the minute.

What sets the movie apart from your average episode of “Without a Trace” is Lehane’s intense sense of place, and Affleck’s willingness to bring his version of working-class Boston to the screen. Like the film version of Mystic River, this one has lots of dark tones, peeling clapboard houses and vowels you could cut with a knife. But it’s less melodramatic, without the histrionics of a Sean Penn. Instead we get subdued but powerful performances from actors such as Amy Ryan, as the missing girl’s addict mom. She’s a textbook “unfit mother,” but she has the charisma of the cocky and slightly insane, and when she tears up about her daughter, so do we. Ed Harris, playing a control-freak detective whose character slowly unfolds before our eyes, could walk out of this with an Oscar nomination.

As a director, Affleck mostly sits back and lets the actors and Lehane’s juicy dialogue do their work. But he uses little tricks, like slowing down time in one scene, to make this a film in which every gunshot unleashes a bit of chaos on the world.

In its last third, the plot twists and turns in directions that aren’t always plausible. But they allow Lehane to pose tough questions that may have people arguing on their way out of the theater — and not just about whether this flick negates the Gigli debacle. At the beginning of the movie, Kenzie recalls his priest’s advice for remaining good in a world full of evil-doers: “Be wise as serpents and innocent as doves.” By the end, we know those two qualities can never be combined.