"13 Going on 30"

The 30-year-old Jenna (Jennifer Garner, of "Alias") is an extremely successful professional -- she's the powerful editor of "Poise," the magazine adored by the 13-year-old Jenna -- with an enviably mod Manhattan apartment and a studly-handsome New York Ranger for a boyfriend. What Jenna doesn't yet know is that to get to where she is today, she has been, to use a term that hadn't yet been invented in 1987, a biatch. One of the cool girls circa 1987, Lucy (Judy Greer, whose performance has the sugary sharpness of a SweeTart), is now her best friend and colleague at the magazine. Apparently, they've both climbed the ladder by stomping on the fingers of everyone around them.

But Jenna doesn't realize that: As a 13-year-old suddenly poured into 30-year-old's body, a naif plunked down in an unfamiliar universe, she's friendly and eager to fit in, and the people around her, especially Lucy, can't figure out what's wrong with her. As the grown-up Jenna, Garner isn't as brilliantly funny as Jamie Lee Curtis was in "Freaky Friday." But her timing has a pleasing loopiness, and she goes just far enough with the puzzled-innocence routine: When she's confronted with a problem she can't wrap her brain around (such as why her sexy boyfriend wants to show her his "thing"), she squeals or furrows her brow just as a 13-year-old would.

Garner also has fun with the physicality the performance demands: Her walk is a young teenager's slightly bowlegged gait, and her rump sticks out like a twitchy bubble -- she hasn't yet learned the grown-up, womanly art of tucking it in. It doesn't hurt that the first outfit we see her in consists of a tiny magenta silk nightie, silver high-heeled shoes, and a sparkly purse -- and this is a work outfit. The outlandish getup is a nod to the way our teenage selves couldn't wait to escape our parents' supervision and wear whatever we please. (This is before we have to reckon with one of the great disappointments of adulthood: That there are very few occasions for truly fabulous outfits.)

"13 Going on 30" is filled with giddy time-warp jokes that work almost in spite of themselves: Grown-up Jenna has missed all the pop-culture developments of the past 17 years and doesn't know, for example, who Eminem is. When a frantic assistant informs her he's on the phone and he wants a decision immediately, she sputters, "Plain! No, peanut!" Then she walks into her office for the first time and sees an array of autographed mementos on the wall, including a "True Blue"-era portrait of Madonna that's been lovingly inscribed to her. "I'm friends with Madonna?" she marvels to herself, half under her breath, amazed by this fabulous life she has lucked into.

Next page: A happy-go-lucky path toward unadulterated niceness