In his feature directorial debut, Ben Affleck shoots an arrow through a valentine to the toothless, pockmarked, drunk, violent and malnutritioned in his native Boston in Gone Baby Gone.

While Affleck pulls back on mythical neighborhood symbolism, Baby isn’t as operatically grand as Mystic River, also adapted from a Dennis Lehane novel about a girl gone missing.

Still, it’s an impossible film to shake, distinguished by Casey Affleck’s dynamic lead performance and the emotionally exhausting moral ground he covers. Seeking absolution for what Patrick Kenzie, P.I., does while looking for Amanda McCready is as crucial to him as finding the little girl. With all of Patrick’s guilt, Baby is very Irish, very Catholic and unafraid to use its story to raise questions of parental preparedness and provoke discussion as to whether his final choice is right.

Patrick is so embedded in his world that he even knows the high-school sexual history of Amanda’s mother, Helene (Amy Ryan). Living in a zoned-out haze of drugs, booze and sex, Helene is a mother only as a byproduct of beer goggles and a bad decision. The last thing Helene seems genuinely concerned about is a child, possibly snatched as collateral in a deal gone bad with a Haitian drug dealer. But Ryan (so good, and so different, as a sweet cop on The Wire) traces deep psychological scars where parental instinct, abandonment issues and impulsivity strangely collide.

Frustrated with Helene’s inactivity, and with dead ends from a police task force (led by Morgan Freeman), Helene’s sister-in-law, Beatrice (Amy Madigan), turns to Patrick and his girlfriend/assistant Angie (Michelle Monaghan). Patrick can use his street connections, on both sides of the law, to gain access where the cops cannot. Aided by two detectives (Ed Harris, John Ashton) serving as department liaisons, Patrick and Angie uncover copious secrets and betrayals.

Ben Affleck has a good feel for the disorientation of gunplay, with unexpectedly loud roars and a couple of confrontations that erupt into broad daylight so as to be even more jarring. But the film’s uneasy energy rests with Casey, whose Patrick can only avoid making a tragic decision for so long.

Patrick lets loose plenty of tough talk in low-lit lounges and bars, like the jovial, lyrical profanity of Good Will Hunting with an edge of danger. Although Patrick’s on the up and up, there’s an unknown, implicit reason he isn’t fazed at the wrong end of a gun. He’s been Amanda, a kid washed up and wasted by birthright alone, and whether a similar second chance could empower her is a close-second priority to finding her alive. Casey Affleck’s wowing work follows the downward spin of a man fated, no matter what, to make an irreparable mistake at trail’s end.

Ben Affleck and co-writer Aaron Stockard adeptly graft Patrick’s regional patter into the story, but not Lehane’s mystery. Of course, the novel had a luxury of not co-starring Freeman or Harris, whose onscreen guilt generally grows with each passing minute they’re absent from a movie. Most nagging suspicions, worse the more you ponder them, turn out to be right in a clunky climax.

The biggest, and best, red herring is that all those twists are just a hook on which to hang Ben Affleck’s most resonant project on either side of the camera since Changing Lanes. Speaking eloquently to notions of sacrifice, Baby’s conclusion is bittersweet, ambivalent and unforgettable.