“Keep your mind on books, not boys,” Beverly Donofrio’s (Drew Barrymore) police chief father tells her early in “Riding With Cars With Boys.” If only she had listened. Instead, she tries to do both and ends up saddled with an infant son and drunken husband at age 15. Her dreams of attending New York University shattered, she settles for a working-class life in Wallingford, Conn., instead.

“Riding In Cars With Boys,” based on Beverly Donofrio’s memoir, might have benefited from similar advice regarding focus. Despite the able direction of Penny Marshall (“A League of Their Own,” “Big”) and an all-star supporting cast, the film misses its mark by trying to tackle too many big-ticket issues for a two-hour movie.

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This ambition focuses the film too much on plot, thus robbing the actors of shining moments. Lorraine Bracco of “The Sopranos” as Beverly’s mother and Academy Award-winning James Woods as her police chief father receive too little screen time to develop as characters, making the parent-child relationship seem synthetic.

The plot follows Beverly from a not-so-innocent but motivated 15-year-old high school student to a hardened 35-year-old single mother. When she is impregnated by well-meaning but dull Ray (Steve Zahn) after “parking” in cars with boys, her vision for her life changes, and she must continually adapt to the hardships of teenage and then single-motherhood. She rears a loving but resentful son, Jason (Adam Garcia), who himself grows up to be a student at NYU.

Barrymore, in undoubtedly the most challenging role of her career, ages two decades in the course of the film. In the high school scenes, she is gawky but charming. As she grows into adulthood, she shows Bev’s strengths and faults without creating a caricatured mood-swing queen. And, despite Barrymore’s immediate likability, her nuanced performance robs us of our sympathies for Bev as her self-centeredness is revealed toward the film’s end.

But none of the characters in the film is entirely without redeeming qualities. Even Ray, Bev’s shiftless, drunken and, it later turns out, drug-addicted husband has moments when the viewer can’t help but like him. This is due partly to the script, penned by Morgan Ward, but mostly to Steve Zahn’s gutsy and honest portrayal of a well-meaning but incapable man.

Known for playing dull tacks with comedic flair (“Happy, Texas,” “Saving Silverman”), Zahn here gives a subtle performance that highlights Ray’s love for his son, devotion to his wife, and helplessness against his addictions. He draws our pity, not because he is a junkie or because he ultimately turns into a filthy middle-aged man living in a trailer, but rather because we see his desire to do better but inability to make good on this.

While Zahn and Barrymore turn in strong performances, their skill is not enough to save the movie. Penny Marshall takes risks with her direction, and many of the actors give heartfelt performances. This amounts to a brave film — one that evades Hollywood sugarcoating — but not a satisfying evening at the picture show. Because the portrayals of Beverly and Ray, among others, are so honest, we find that there is no one with whom we can sympathize. Though we begin to feel for teenage Beverly, her self-absorbed adult persona immediately turns us away, denying us that connection we need in order to accept her character flaws.

The film’s fatal flaw is that it attempts to tackle too much and in doing so, gets off track. “Riding in Cars With Boys” touches on serious issues but fails to probe them in depth. It asks us to consider the effects of the Vietnam War, the devastation of drug addiction, and the impact of poverty on children.

Yet in attempting to cover so much, the film instead frustrates in its superficial examination of weighty topics. Perhaps Donofrio’s story should have been left to the original memoir, as that is a better format for delving into such an array of issues. Though “Riding in Cars With Boys” illuminates the difficulties of teenage pregnancy, its glossing over her other issues leaves us empty and dissatisfied.

We should applaud “Riding in Cars With Boys” for resisting the temptation to provide a typical chick flick by ignoring the realities of life, but this originality and freshness are not enough to make a mediocre movie great.