Starring Morgan Freeman, Casey Affleck, Michelle Monaghan, and Ed Harris. Rated 14A.

Gone Baby Gone deftly explores the ethical repercussions of a four-year-old girl's mysterious disappearance on Boston's seedy south side. Based on Dennis Lehane's novel, it's the most compelling big-screen portrayal of Boston-based lowlifes since the 1973 cult classic The Friends of Eddie Coyle. There's enough unexpected twists in Gone Baby Gone the kind of story that comes complete with a thin layer of grime to keep everybody guessing. However, the biggest surprise of all may be Ben Affleck's self-assured debut as a mainstream director.

His increasingly pathetic acting career is enough to make anyone forget that he was once a Hollywood golden boy, winning an Oscar almost a decade ago for cowriting Good Will Hunting with Matt Damon. As co ­writer, with Aaron Stockard, for this screen adaptation of Lehane's book, Affleck serves up a nimble script that for all its grit is essentially a character-driven morality play. For Affleck, this project was obviously both a labour of love and a chance to redeem his self-confessed reputation as an actor who grabs the cash and runs. In perhaps the most sensible choice of his career, he stays firmly planted behind the camera.

Another smart move was to cast his brother Casey in the lead. The younger Affleck, clearly the family member who inherited the lion's share of acting chops, brings an appealing combination of street smarts and vulnerability to his role as private detective Patrick Kenzie. Along with his girlfriend and professional partner, Angie (Michelle Monaghan), Kenzie runs a modest business from his home. Despite reservations, the couple latch on to the hottest case in town after the relatives of a missing girl hire them to augment a flagging police investigation.

Kenzie is in for a rough ride: the crusading police commissioner (Morgan Freeman) views his participation with skepticism, and the head detective (Ed Harris) is likewise opposed. Ben Affleck's skill in putting together this story of sin and redemption should give his career the kind of boost that could only happen in Tinseltown.