Did you hear about the actor-turned-director who took an acclaimed Dennis Lehane novel and nailed both its distinctive South Boston atmosphere and its exploration of impossible moral choices? Hint: It’s not Clint Eastwood.

There are those who still insist that Eastwood’s 2003 adaptation of Lehane’s Mystic River was an exceptional piece of filmmaking, and that its multiple Oscars somehow vindicate this notion. But despite his reputation for restraint, Eastwood piled on the operatic music and melodramatic staging, and indulged Tim Robbins’ hideous, inconceivably acclaimed performance as a guy wearing a big “I’m psychologically damaged now! Ask me how!” sign around his neck.

You may snicker at the notion of Ben Affleck—Ha, ha, Gigli! Ha, ha, Bennifer!—turning serious filmmaker, but whatever you think of him as an actor or a public personality, the dude does a lot of things right in his riveting adaptation of Lehane’s Gone Baby Gone. The Boston native gathers a terrific cast and crafts a portrait of its world as riveting as it is heartbreaking.

The plot launches from the disappearance of 4-year-old Amanda McCready (Madeline O’Brien) from her bedroom in the Dorchester neighborhood. Police and the media are all over the story of the panicked mother Helene (Amy Ryan), but Helene’s brother Lionel (Titus Welliver) and sister-in-law Bea (Amy Madigan) hire private investigator Patrick Kenzie (Casey Affleck) to help with the investigation. Patrick’s native sense of the neighborhood allows him to make headway, discovering that Helene’s nasty habits may have provided a motive for Amanda’s disappearance. And the more Patrick and his girlfriend/business associate Angie (Michelle Monaghan) dig into the case, the harder it may be to emerge unscathed.

From virtually the opening shot, Affleck makes it clear how firmly he’s going to ground this story in its sense of place. He presents a parade of background figures whose less-than-Hollywood-beautiful faces and bodies show the consequences of hard living. He allows them to speak not just with a recognizable accent, but in the language of a lower-class neighborhood—with pre-teen kids on bikes shouting out “f—k your muthah,” and a woman talking on television about organizing a candlelight “vij-oo-al” for the missing girl. Unlike so many literary adaptations that focus almost entirely on plot, Gone Baby Gone demonstrates an intrinsic understanding that the setting is as fundamental a part of the story as any of the characters.

And the characters themselves don’t exactly suck, either. Casey Affleck nails the combination of quick mind and street-smarts that makes Patrick more dangerous than people expect just from looking at him; Welliver does terrific work as a blue-collar guy whose demons make him want to be a guardian angel; even Morgan Freeman takes a break from conveying noble gravitas in his role as an emotionally wounded cop. But the most remarkable performance comes from Amy Ryan, who invests the weak, pathetic Helene with such humanity that it’s impossible not to find her fascinating even as we find her actions horrifying.

As it turns out, the role is absolutely critical to Ben Affleck’s exploration of Lehane’s dark moral terrain. Like Mystic River, Gone Baby Gone deals with the long-range consequences of children mistreated; here Lehane addresses not just the kids themselves, but with the adults whose “won’t someone please think of the children” decisions aren’t always as simple as they appear. Affleck auteur does an impressive job of portraying people making the wrong choice for the right reason, because the world in which they’re asked to make those choices don’t offer many alternatives that are obviously “right.”

It takes a while for Gone Baby Gone to come to those devastating conclusions, and along the way it becomes a fairly straightforward and familiar crime procedural that loses track of Monaghan’s character along the way. The plot takes some improbable turns, and Affleck responds with rote measures like extended flashbacks to put all the pieces together. But any directing blunder he might possibly have committed feels forgivable after the staggering impact of his final shot. It’s a simple, insinuating encapsulation of so many different ideas: one man whose decision has given him a life-long responsibility; the tragedy of emotionally abandoned youth; a sense of how little changes in a neighborhood like Dorchester from one generation to the next. There’s both familiarity and deep sadness here—the kind that comes from someone who’s talking about his own back yard.

GONE BABY GONE

***1/2 (three and a half out of four stars

Starring Casey Affleck, Michelle Monaghan, Ed Harris.

Directed by Ben Affleck

Rated R.