The story in “Conviction” is indeed told with conviction. There isn’t much more to say about it than that. It’s a true story, it has underdogs fighting against the odds, and its message is noble and worthwhile. I found it to be generally engaging and well-acted. And I have no interest in ever watching it again.

Hilary Swank puts on her steeliest jaw and stiffest upper lip to play Betty Anne Waters, a working-class Massachusetts woman in the 1980s who grew up poor and often in foster homes. Her best friend since childhood has been her brother, Kenny (Sam Rockwell), a life-of-the-party screw-up whose youthful fondness for mischief and light felony has not diminished with time. (I can tell this is a true story because that description fits just about every real-life “Kenny” I’ve ever known.)

Kenny has had enough run-ins with the law to be on a first-name basis with many of the personnel at the town’s police station, but his offenses have typically been minor. UNTIL HE BRUTALLY STABS A LADY TO DEATH!! That’s what he’s charged with, anyway. He insists he didn’t do it but, in 1983, is convicted of “first-degree murder with extreme atrocity” (which makes it sound kind of awesome) and given a life sentence.

Betty Anne, convinced her brother is innocent but having exhausted her limited resources to hire lawyers to defend him, embarks on an incredible mission: She will become a lawyer and fight for Kenny herself. This means getting her GED, then earning a college degree, then graduating from law school, then passing the bar exam. This is a multi-year plan, obviously. If the film were fictional, you’d never believe that a character would do it.

Betty Anne is a single mother for much of the time, her marriage having failed as the result of her single-minded devotion to saving her brother. She’s supported by a law-school friend named Abra (Minnie Driver), who’s funny and grounded and helps keep Betty Anne sane.

The film makes smart use of childhood flashbacks to establish the bond between brother and sister (the young versions are played by Bailee Madison and Tobias Campbell), and to provide insight into how they came to be as messed-up as they are without being melodramatic about it. It’s all the usual reasons: absent dad, neglectful mother, not enough money, etc.

Betty Anne’s determination is based on her certainty that Kenny is innocent. She is sometimes the only person convinced of it. Even the audience is not always entirely sure Kenny didn’t kill that lady. He’s kind of a dirtbag — a likable one, thanks to Rockwell’s winning performance, but a dirtbag nonetheless — and we’ve seen that he has a violent streak. No one in town would have thought he’d commit murder, but no one seems terribly shocked by the idea either. Kenny himself says, “It doesn’t matter whether I killer [her] or not. I’m a piece of s*** anyway.”

The director is Tony Goldwyn, with from a screenplay by Pamela Gray (who also wrote Goldwyn’s directorial debut, “A Walk on the Moon”). Goldwyn is better known as an actor than a filmmaker, which may explain why “Conviction” feels more like an acting showcase than a film. Swank, Rockwell, and Driver each get a meaty scene or two, with Swank delivering a fine Massachusetts accent; Melissa Leo plays a grudge-minded cop; Peter Gallagher is a crusading lawyer who helps reexamine the old evidence; Clea DuVall and Juliette Lewis are memorable as trashy women from Kenny’s past who testify against him.

Taken separately, almost every element of the film is praiseworthy. But as a whole it feels perfunctory and flat, like some unquantifiable part of the movie-magic process just didn’t fire the way it’s supposed to. It’s an earnest and well-produced movie, though, even if it lacks that special spark.

B- (1 hr., 47 min.; R, abundant harsh profanity, brief violent images, a guy's butt.)