Cast anyone but actress savant Natalie Portman as the pregnant, white trash teenager axis of "Where the Heart Is," and this warm-fuzzy soap opera of stock crises and Hallmark card moments would be pretty close to insufferable.

Propelled to the big screen only on the momentum of the novel's Oprah Winfrey book club endorsement, even with Portman -- who by carrying this movie proves absolutely her astounding talent -- in the lead, this low-impact unwed motherhood epic never gets any deeper than a pebble skipping across a pond.

That pebble is Novalee Nation (Portman), a near-illiterate 17-year-old abandon in the parking lot of a Sequoyah, Oklahoma, WalMart by her rat bastard boyfriend when they were supposed to be moving to California together in his $80 car.

A resourceful and stylish kind of loser (half her sparse wardrobe is flirty, spaghetti-strapped sun dresses), Novalee lives clandestinely in the WalMart for the length of a few upbeat, triumph-over-adversity musical montages until her baby is born -- at which point her minor celebrity (TV crews park outside her hospital window angling for a story on the WalMart Mommy) helps net her a job at the store where she'd been sacking out for six weeks.

The pandering and meandering screenplay by Lowell Ganz and Babaloo Mandel -- whose bad work ("Father's Day," "Multiplicity") far outweighs their good ("EDtv," "A League of Their Own") -- spends the next two acts testing Novalee's character over a span of five years with soft-peddled episodes of melodrama (a kidnapping, a tornado), all resolved with little more than a nice big hug.

The endearing Portman's earnest, supple and probably too-graceful performance as the rapidly-maturing Novalee is just about the only heart in "Where the Heart Is." She's a joy to watch, even when the film's facade is falling down around her.

Although the supporting cast is a strong one, many of the characters are little more than only-in-the-movies plot pointers and emotional cheerleaders who dumb-luck into her life, like randy retiree Stockard Channing as the mother figure who takes her in and Keith David ("Pitch Black") as a photographer who becomes her career mentor after a chance meeting at his one-day-a-month portrait gig at the WalMart.

Sally Field leaves a lasting impression in a scenery-chewing cameo as Novalee's long-absent, peroxide blonde tart of a mom in a bellybutton top and white pants so thin you can see her undies. But her presence in the film serves no purpose. Apparently she drove all the way from New Orleans just to swipe $500 from a daughter she hasn't seen in 10 years.

More significant roles go to Ashely Judd, who plays a peppy, big-sisterly nurse with six kids and even worse romantic judgment than Novalee, and James Frain ("Reindeer Games") as a scruffy, stressed-out, doormat of a librarian who pines pathetically for our heroine throughout the entire picture.

One of the first signs that the movie isn't going to be remotely in touch with the reality comes when this milksop -- who had shadowed her and was lurking outside the locked and darkened WalMart -- hears her screams as she goes into labor and jumps through a plate glass window to deliver her baby. Excuse me?

Directed by first-timer Matt Williams, "Heart" seems not at all interested in exploring Novalee's psyche to discover why she's has such a dysfunctional life in the first place. He instead spend huge chunks of screen time following the ex-boyfriend around just so we can get some gratification out of watching him blow a shot at country music stardom and become a regret-filled addict and cripple as comeuppance for his mistreatment of our sweet girl.

In a big rush to see Novalee blossom and succeed, Williams also has a bad habit leaping right over the very times in which we could watch her grow just so he can get to celebratory moments of emotional prosperity shot through golden lens filters.

Yet Portman emerges from this picture smelling like a rose. She knows her character's nuances even if the screenwriters and director don't and the fact that audiences won't be snickering loudly through the whole thing can be attributed entirely to her.