Move over, Julia Roberts. Jennifer Garner has a smile that's a big, lush, wide-screen dazzler, with voluptuous candy lips that nudge her face into dimples as lovely as they are long. I'm not sure if Garner has grinned three times on ''Alias,'' but in 13 Going on 30, she takes on the role of Jenna, a scrawny, MTV-addicted nerd who finds herself occupying a woman's body, and she has an enchanting yet friendly radiance. She wraps the audience in an embrace of lost innocence.
The movie, directed by Gary Winick (''Tadpole''), has been conceived, quite unabashedly, as a feminine variation on ''Big,'' and given what a hilarious and enthralling pop concoction that 1988 Tom Hanks comedy was, I didn't expect lightning to strike twice. But it does. ''13 Going on 30'' is a bit glossier than ''Big,'' with more obvious fudges in fantasy logic; Jenna, after freaking out at the hunky stranger emerging from her shower, slips into her new adult role a shade too easily. (She walks around in heels as if she'd done it for years.) Yet once the movie gets going, it's funny, resonant, and magical in the same way ''Big'' was, and you can pinpoint the moment in it when Garner becomes a star.
Jenna, who starts out in 1987 as a 13-year-old in love with Rick Springfield, doesn't just pop into a bigger body, like Hanks in ''Big.'' After getting doused with wishing dust, she wakes up to discover that 17 years have passed and that she is living the life of her future 30-year-old self. She is now a tall, beautiful high-maintenance princess who dates a New York Ranger, occupies a Fifth Avenue apartment brimming with designer dresses, and works as one of the trendy top editors at a women's magazine called Poise. She gets to use a limo; she's even drinking buddies with Madonna! Yet as thrilled as she is at all this, she mostly just wants to dance.
As an office party winds down, Jenna attempts to jump-start the evening by leading her coworkers in the demons-do-the-hustle chorus line from Michael Jackson's ''Thriller'' video. These are moves she generally confines to her teenage bedroom: the claws in the air, the twirls and hand claps and head shakes. For a moment, everyone thinks that she's nuts, and she does indeed look a bit foolish -- at least, until you hook into the vibe of eager pleasure radiating from Garner's eyes, her movements, her beaming grin. There's nothing suave or measured in her attitude; she dances with the free heart of an adolescent who hasn't yet learned how to be cool. That's the beauty of the scene: Because of the deep uncoolness -- the sincerity -- of her joy, you can't stop watching her. It's no surprise when everyone joins her on the dance floor.
The title ''13 Going on 30'' is more descriptive than it sounds. As a girl, Jenna, played by the perky Christa B. Allen, longs, in her insecurity, to be invited into the Six Chicks, a clique of haughty, duplicitous baby vixens. It's not just that they're the in crowd; they're fashion-victim bitches far beyond their years. They've turned innocence into a dirty word. But the moment that Jenna is ''reborn'' as her adult self, working in the spangly cutthroat atmosphere of Poise, she finds herself in an office driven by that same catty spirit of one-upmanship -- an office ruled by overgrown teen snobs. Her life, she discovers, took a turn: She joined the Six Chicks after all, and this is what she became. ''Big,'' in its way, tapped the spirit of a new generation of high-fiving male hedonists who still felt like kids beneath their tailored yuppie facades. ''13 Going on 30'' has a different resonance. It's saying that the postfeminist princess culture has become a trap for every age group, ensnaring girls and women alike in a perpetual chain of arrested Babe Values.
The journey of the movie is Jenna's discovery of that trap from the inside. Garner's performance isn't a full-scale out-of-body burlesque, like Jamie Lee Curtis' in ''Freaky Friday.'' Instead, she cuts out all traces of adult consciousness, of irony and flirtation and manipulation, reducing herself to a keen, goggle-eyed earnestness that's utterly beguiling. Mark Ruffalo is Matt, the grown-up version of the roly-poly teenage friend whom Jenna rejected when she joined the Six Chicks, and Ruffalo, who's like a quizzical teddy bear, brings off something deft: He spends most of the film sizing up Jenna with quiet, brooding distrust -- and melting on the inside anyway. ''13 Going on 30'' also features Andy Serkis -- yes, Gollum himself -- as Poise's madly mincing editor in chief. Only in the movies would a man this silly be running a powerful women's magazine, but then, ''13 Going on 30'' is the rare commercial comedy that leaves you entranced by what can happen only in the movies.