Before Dennis Lehane wrote Mystic River, the basis for Clint Eastwood's Oscar-winning film, he wrote four detective thrillers about blue-collar private eyes Patrick Kenzie and Angie Gennaro, a sort of shanty Nick and Nora Charles. Ben Affleck's directing debut is an adaptation of the fourth and best of the series.

. Little Amanda McCready (Madeline O'Brien) has vanished from her own bedroom in Dorchester, a poor but tight-knit, working-class Irish-American neighborhood. Her mother, Helene (Amy Ryan), is distraught, the community is up in arms, and the police and news media are all over the case. But three days after Amanda's disappearance, the cops have come up empty and the child's frustrated aunt Bea (Amy Madigan) turns up at the modest office of PIs Patrick Kenzie (Casey Affleck) and Angie Gennaro (Michelle Monaghan), her reluctant husband Lionel (Titus Welliver) in tow. Bea pleads with them to join the investigation: Both grew up in Dorchester and might be able to tease out leads the police have missed. Despite Angie's misgivings — an abducted child gone for three days is almost certainly dead, and she dreads the thought of fishing a tiny, battered corpse from a dumpster — they take the case and broker an uneasy truce with Captain Jack Doyle (Morgan Freeman), who heads up the Boston PD's Crimes Against Children unit, and his lead detectives, Remy Bressant (Ed Harris) and Nick Poole (John Ashton). It doesn't take long to establish that the hard-living Helene lied about the circumstances of Amanda's disappearance, and that her heavy-duty drinking and drugging lifestyle probably has something to do with the child's abduction. But the deeper Patrick and Angie dig, the more conflicted they become about what they should do with the hard-won truth of the matter.

Though marred by artificial hard-boiled dialogue, some rocky story transitions and wandering accents, Affleck's first outing as director is an admirably dark, ambiguous thriller with the courage of its morally compromised convictions. Fans of Lehane's Kenzie-Gennaro books will lament the fact that starting with the fourth book means losing the couple's extensive backstory, but the essence of their fragile, damaged bond comes through even if you don't know what shaped it. And Affleck's decision to cast both extras and supporting players from Boston locals pays off handsomely: You don't find faces like that of Jill Quigg, who plays Helene's sozzled, bitterly defensive best friend, in a casting directory.