By Paul Clinton
CNN Reviewer

(CNN) -- "Riding In Cars With Boys" assumes the premise that having a child at the age of 15 can really mess up your life.

Well, duh. And it doesn't get much better from there. The movie, based on a book also titled "Riding in Cars with Boys," left a lot of the book's plot on the cutting-room floor in the transition from the page to the screen.

Drew Barrymore plays Beverly Donofrio, the real-life woman who wrote the book. The plot details Donofrio's struggles in growing up as a teen-age mother in the late 1960s with a child in tow.

The movie, directed by Penny Marshall, begins in the mid-1980s with Beverly as a grown-up woman of 35 riding in a car with a boy. We soon find out that the boy is actually her 20-year-old son Jason, played by Australian actor Adam Garcia. Bev has just finished writing "Riding in Cars with Boys," her autobiography, and she and Jason are on a journey to find his long-lost father Ray, played by Steve Zahn. It seems that she needs Ray's approval of the material before it can be published.

Now we flash back to the '60s, where Bev is a bright teenager living with her stern cop father (James Woods) and loving mother (Lorraine Bracco) in a Connecticut town. Her best friend is Fay (Brittany Murphy of "Don't Say A Word"), and together the two girls face the trials and tribulations of a suburban adolescence.

Disappearing dreams

Spurned by the boy of her dreams at a high school party, Bev finds herself in the arms of a not-to-bright 18-year-old dropout -- the aforementioned Ray. Nature takes its course and Bev finds herself "with child."

The following scenes where Bev attempts to miscarry, and then finally confronts her parents with the truth, are hilarious. Finally, in order to please her parents she marries the bum in an extremely sad-sack wedding. In an impromptu speech at the reception Fay announces that she too is pregnant and the father is heading for Vietnam. She has a girl she names Amelia.

Ray and Bev set up house, and she watches all her dreams disappear one by one. Jason is played by four different young actors (three of whom are brothers) as he grows up. The best are Logan Arens, playing a 3-year-old, and Cody Arens, playing a 6-year-old.

One of the film's greatest weaknesses is the fact that we don't see Jason from age 8 until he's 20 and played by Garcia. That's a huge unexplained gap which weakens the narrative.

Bev's greatest dream is to finish college and become a writer. It's not an easy achievement while raising a kid -- actually two kids, since Ray is such a total loser. At this point, the narrative falls apart again when Ray announces -- out of the blue -- that he's a heroin addict. Boom, he's out of the picture -- literally, until the very end. Zahn is an extremely gifted actor, but he's not good enough to cover that gaping hole in the plot.

More struggles

Now a single parent, Bev continues to struggle, and we get a glimpse of how mother and son are helping each other in their efforts to grow up. But things get fuzzy here as we flash back and forth between the early years and the present day, where Bev and Jason are on their journey.

The book, and getting Ray to sign off on it, are the main motivations driving the story. That's another major flaw in the story, because -- though Bev has a huge emotional investment in the project -- we never see her struggle to write the book or the sacrifices she has to make in order to achieve her goal.

This hole in the plot is just one more casualty of the lack of back story during the 12 years when Jason goes from 8 to 20. This also leaves Bev's life from the age of 23 to 35, a complete void.

On the plus side, Barrymore gives a wonderful, heartwarming performance, and Brittany Murphy proves that her great work in "Don't Say A Word" was not a fluke. Marshall is an expert in conveying human emotions in her films, and that does come through.

Unfortunately, the story doesn't.

"Riding In Cars With Boys" opens nationwide on Friday and is rated PG-13.