“I always thought it was the things you don't choose that make you who you are,” declares the narrator at the opening of this powerful mystery, words that turn out to be hauntingly prophetic. Adapted from a novel by Dennis Lehane (Mystic River), the movie centers on the disappearance of a four-year-old girl whose life has already been sadly defined by her vile single mother and grim working-class Boston neighborhood. The girl's aunt hires a pair of private detectives (Casey Affleck, Michelle Monaghan) to assist the police, and as they get closer to the truth, even the child's rescue begins to seem like a tragic fate. Ben Affleck directed and cowrote the script; his biggest gamble was casting his irksome little brother as a pistol-whipping tough guy, but the picture is so superbly executed in every other respect that Casey seems more quirky than miscast. With Amy Ryan, Ed Harris, and Morgan Freeman. R, 104 min.