April 23, 2004 |  Yes, the child-trapped-in-adult-body gimmick is shamelessly recycled from Big and dozens of other movies. And, yes again, the predictable plot is a standard-issue morality play about the dangers of getting what you wish for. But trust me: It doesn't really matter that 13 Going on 30 is a star vehicle composed second-hand parts. What really counts is the megawattage of the star in the driver's seat.

Much like her character is magically transformed from dweebishly insecure adolescent to glamorously successful adult, Jennifer Garner makes the tricky transition from cult-fave TV action icon to full-fledged, ultra-charismatic leading lady in this sprightly romantic comedy-fantasy. Hype-mongers and trend-watchers doubtless will argue whether she's “the next Julia Roberts” or “the new Sandra Bullock.” But most mainstream moviegoers simply will accept her – no, make that embrace her – as delightfully appealing on her own terms.

Credit co-screenwriters Josh Goldsmith and Cathy Yuspa (who also gave us What Women Want ) for wasting little time while setting up their fantastical premise. The early scenes, set in 1987, briskly establish young Jenna Rink (Christa B. Allen) as a nerdy-cutie waif who dreams of hanging with the hippest girls at her school. Matt (Sean Marquette), her classmate and not-so-secret admirer, advises her to be “an original,” not a conformist. “But I don't want to be an original,” Jenna pointedly replies. “I want to be cool.”

Jenna gets her chance to chill after she's sprinkled with “Wishing Dust” during her disastrous 13 th birthday party. She wakes up to find herself 17 years older – and, more important, drop-dead gorgeous – with a hunky celebrity athlete boyfriend, a lavish Manhattan apartment, and a fabulous job as editor of Poise, a glossy women's magazine.

Trouble is, she's still little Jenna inside her head. She has no idea how she got where she is, what she's done since she's been there, or what she's supposed to do to survive and thrive.

From the very moment she literally rolls out of bed as the cluelessly adult Jenna, Garner wins you over with equal measures of giggly-girlish exuberance and anxious-adolescent befuddlement. It's not terribly surprising to see that, after three seasons of acrobatic butt-kicking on TV's Alias and her Elektra-fying super-heroics in last year's Daredevil , this fetching actress is exceptionally adept at physical comedy. What is surprising, and delightful, is the full-tilt energy and resourceful expressiveness she brings to conveying the illusion of an ungainly adolescent trapped inside a mature adult's body.

Garner throws herself so fully and effectively into her role that she occasionally appears to be radiating emotions with every part of her body. Indeed, in a few key scenes, she vividly conveys Jenna's high spirits and giddy pleasure through the graceful curling and uncurling of her toes.

In this kind of movie, of course, the protagonist always comes to regret having a wish fulfilled. And, sure enough, Jenna learns she has risen to the top by being ruthless and demanding – qualities, it should be noted, that are greatly appreciated by her prissy editor-in-chief (Andy Serkis, only slightly less animated here than in the Lord of the Rings trilogy). Her closest associate, Lucy Wyman (Judy Greer), just happens to be the conventionally grown-up version of her snootiest hip-chick classmate. And while Matt (Mark Ruffalo, establishing romantic-comedy leading-man cred) has grown up to be an amiable and attractive photographer, he's decidedly aloof when Jenna tries to reconnect with him – largely because, the last time they met as teens, she banished him from her life for being terminally uncool.

Director Gary Winick ( Tadpole ) does a nifty job of bringing a fresh spin to most of the script's clichés, and emphasizing the nuggets of emotional truth provided by writers Goldsmith and Yuspa. 13 Going on 30 only gradually reveals how much Jenna won and lost by transforming herself into her ideal of adult success, suggesting the scenario could have been played more seriously with thought-provoking results. (What if, instead of pivoting on magic, the movie had dealt with a discontented woman who psychologically blocks all memories since her innocent adolescence?) The happily-ever-after wrap-up is dramatically satisfying and commercially sound – yet also something of a let-down. But never mind: Garner is so immensely likeable here that you can't help wanting everything to turn out just peachy for her character.