When a movie says it's "based on" a true story, all too often it means that after the script doctors get through with it, what's left is too predictable and packed with clichés to bear any resemblance to the randomness of real life. Such is the case with "Riding In Cars With Boys."

But it just so happens that clichés and predictability are director Penny Marshall specialty. Idle since "A League of Their Own" -- which was totally trite yet thoroughly enjoyable -- Marshall applies her syrupy, low-cal sentimentality to this adapted autobiography of writer Beverly Donofrio, whose youthful ambition was derailed in 1965, by getting knocked up at age 15.

A maudlin but self-deprecating, bittersweet comedy-drama in which major crises are solved with little more than hugs, Beverly's journey through motherhood would be the stuff of a Lifetime Channel movie-of-the-week if not for its gusty sense of humor and a phenomenal performance of extraordinary depth and range by the previously beguiling but frivolous Drew Barrymore.

Rising above the contrivances around her, the actress plays Beverly from 15 to 35 and is completely convincing in her every gesture of both age and parentage. As a too-young mother in the '60s, unhappily married to the good-hearted but irresponsible oaf that impregnated her (Steve Zahn), she is immature but beholden to her infant son, melancholy but determined to persevere. She wants to get her GED, go to college and become a writer, while remaining a good mother. Every fraction of her frustration, every pang of guilt over sometimes wishing her son hadn't been born, every motherly instinct, every moment of pride, and every giddy girly urge of her still-youthful soul -- all of these emotion play persuasively on Barrymore's face.

As the mother of the college-age Jason (Adam Garcia) in the '80s scenes -- sprinkled throughout the film for contrast and narration -- Barrymore is even more remarkable. She obliges her director's desire for motherly stage business (habitually fixing her son's cowlick over his protests), but what drives her performance are the subtle manifestations of 20 years of parenting. She'll pat her son absentmindedly, argue with him in habitually fussy tones of voice, or shake her head in frustration in a dismissing manner that suggests she's shaken her head at him 10,000 times before. Her entire cadence is that of a woman aged by her life of struggle but proud of how she's survived and raised a good son.

These 1980s scenes -- which take place on a road trip to see Jason's father for the first time since he left in the early '70s -- are a transparent concoction of screenwriter Morgan Upton Ward ("A Pyromaniac's Love Story") to build the story toward therapeutic closure. But the bulk of the film is taken from Donofrio's book and feels true in spirit, if not in fact, as she raises her son through several stereotypical episodes of his life. Scripted to highlight Beverly's parental learning curve and demonstrate how she came to be a loving but controlling mom, these scenes of tears and laughter are often forced. But their prefabricated nature just serves to highlight Barrymore's extremely natural characterization all the more in contrast.

The recreation of the 1960s is quite vivid, as are the complexities of Beverly's relationships with her bitterly disappointed father (James Woods) and the husband she never really loved no matter how hard she tried. In the thankless role of designated loser, Zahn gives one of his best performances too. Convincingly spanning the same age range as Barrymore, he deftly injects just enough humor to garner a little sympathy while becoming a drunk, then a junkie before Beverly throws him out of the paint-peeling shack of a home he bought with high hopes at the beginning of their teenage marriage.

But Penny Marshall's complacency toward the picture's banality is a serious drag on its emotional sincerity. The best moments are incidental (caught by her pop during a heavy petting session, teenage Beverly digs herself deeper into trouble by trying to explain she never goes past second base on a first date). It's the scenes designed to be memorable or pivotal that are the most hackneyed. Because Marshall never strays from formula, there isn't a single moment in the entire film that the audience can't anticipate two or three scenes in advance.

Barrymore almost heroically carries "Cars" through its unrelenting unoriginality, making this fictionalized memoir a genuinely warm and fuzzy charmer in spite of its corniness. But the specter of what the movie would have been without this outstanding performance haunts every moment of its self-indulgent 123-minute run time.