Casey Affleck follows up his mesmerizing turn as Jesse James' once-idolatrous killer with an equally superlative, and quite different, performance in brother Ben Affleck's highly auspicious directorial debut, "Gone Baby Gone" (Miramax).

Adapted from the novel by Dennis Lehane of "Mystic River" fame, this is a hard-hitting look at the seedy underworld of the blue-collar Dorchester area of Boston.

Affleck plays private investigator Patrick Kenzie, who, in tandem with his girlfriend-partner Angie Gennaro (Michelle Monaghan), agrees to help find missing 4-year-old Amanda, whose Aunt Bea (Amy Madigan) begs him to help supplement the police's thus-far unfruitful efforts. (Patrick and Angie headline a series of books by Lehane.)

Bea's ex-con husband, Lionel (Titus Welliver), is the brother of Amanda's slatternly mother, Helene (Amy Ryan), who, we soon learn, is a serious drug addict who had not -- as she originally stated -- been merely at a neighbor's house when the child disappeared, but was hanging out at the local bar for hours with her drug cronies.

The police on the case -- Capt. Jack Doyle (Morgan Freeman), head of the Crimes Against Children unit, whose own child had been kidnapped years before, and tough Det. Remy Bressant (Ed Harris) -- view the baby-faced Patrick with skepticism. But he soon proves his mettle by uncovering facts of which they were unaware. (When he goes to the aforementioned bar to determine Helene's actions on the fateful night, he handles the truly frightening ruffians there with fierce aplomb.)

Patrick and Angie fearlessly infiltrate the scuzzy drug underworld, going up against drug lord Cheese (Edi Gathegi) and other questionable types, gradually raising his stock with Doyle and Bressant.

The plot takes several surprising turns, which we shouldn't spoil, but suffice it to say the ending will definitely give audiences something to chew on.

Ben Affleck displays total assurance in telling this hard story (he co-wrote the screenplay with Aaron Stockard).

Though Freeman, Harris, Monaghan and the others deliver their usual quality work, it's Casey Affleck, as in "The Assassination of Jesse James by the Coward Robert Ford," who commands your interest. Also outstanding is Ryan, whose hard-bitten, foul-mouthed mother makes a strong impression. The working-class types who make up the South Boston scenes are very well cast, some actually local nonprofessionals.

Interestingly, the film opens with Patrick quoting his priest on how "you could get to heaven and still protect yourself from all the evil in the world," to which the priest replies with the famous passage from Chapter 10, Verse 16 of St. Matthew's Gospel: "Be wise as serpents, yet innocent as doves."

Though the seedy environment, pervasive expletives and sporadic but graphic violence will not be to everyone's taste, mature viewers will find those elements and some morally troublesome actions -- the spontaneous killing of a bad character and the film's provocative conclusion -- handled with gravity and intelligence.

The compassionate view toward abused children that permeates the story is another plus. And the film also provides a worthy service in helping raise awareness of the frequency of child abductions, which comprise 85 percent to 90 percent of all the disappearances of children.

The film contains pervasive rough language, profanity, violence with blood, corpses, an impulsive vigilante-style killing, drug use, implied nonmarital relationships and child abuse references. The USCCB Office for Film & Broadcasting classification is L -- limited adult audience, films whose problematic content many adults would find troubling. The Motion Picture Association of America rating is R -- restricted. Under 17 requires accompanying parent or adult guardian.

Movies have been evaluated by the U.S. Conference of Catholic Bishop's Office for Film and Broadcasting according to artistic merit and moral suitability. The reviews include the USCCB rating, the Motion Picture Association of America rating, and a brief synopsis of the movie.

The classifications are as follows:

• A-I -- general patronage;
• A-II -- adults and adolescents;
• A-III -- adults;
• A-IV**
• L -- limited adult audience, films whose problematic content many adults would find troubling. L replaces the previous classification, A-IV.
• O -- morally offensive.