Jennifer Garner takes an appealing departure from her action-heroine roots in "13 Going on 30," a "Big" for the Jane magazine set.

We meet Jenna Rink (played as a teenager by Christa B. Allen) the day before she turns 13. With a dorky retainer on her teeth and Kleenex stuffed in her bra, Jenna is pretty much the typical 1980s adolescent girl, mooning over Rick Springfield, learning the steps to Michael Jackson's "Thriller" video by heart and getting dissed (did they say "dissed" in 1987?) by the popular clique at school. All Jenna wants is to belong to this exclusive group, known as the Six Chicks, to the consternation of her best friend and next-door neighbor, Matt. He dismisses the lip-gloss-wearing, insult-hurling, Madonna-listening vixens as totally unoriginal, but Jenna is adamant: "I don't want to be original!" she wails. "I want to be cool!"

That plea winds up being answered when after a disastrous birthday party, Jenna closes her eyes and wishes she were "30, flirty and thriving," a cover line from her favorite magazine, Poise. When she opens her eyes, Jenna, now played by Garner, is indeed 30 -- and apparently flirty, as a well-built hockey player pads through her apartment to take a shower. The apartment, a gorgeous one-bedroom in Greenwich Village with an entire closet full of shoes, indicates that the "thriving" part has come to pass, too.

Less interested in her furniture than in the breasts that have appeared without the benefit of Kleenex or other paper products, Jenna stumbles into what seems to be her life 17 years hence: She's now an editor at Poise, as is her current best friend, Lucy, the former leader of the high school pack. Utterly confused, Jenna gets in touch with Matt, who has become a photographer in Manhattan and hasn't spoken to his old crush in years. Slowly, it becomes clear that the sweet, bumbling 13-year-old has turned into the 30-year-old Six Chick from Hell.

If "13 Going on 30" isn't exactly original, it's still reasonably cool, thanks to Garner's uninhibited portrayal of a nerd trapped in a "Sex and the City" body, and to the inspired casting of Mark Ruffalo as the grown-up Matt. Best known for his portrayal of the wastrel brother in "You Can Count on Me" and recently seen in "Eternal Sunshine of the Spotless Mind," Ruffalo brings a yearning soulfulness to his portrayal of the spurned friend. His seriousness makes the romantic payoff affecting, if not unexpected.

With her girlish dimples and open, friendly face, Garner is clearly cut out to be America's next Sweetheart; she has the same magic mix of allure and accessibility that the job calls for (think of Meg Ryan, Julia Roberts and Sandra Bullock). And she swings right along with the humor of "13 Going on 30," dutifully putting her name at the top of a piece of notebook paper at her first Poise editorial meeting and, when asked what to tell Eminem when he calls, nervously answering, "Plain! Peanut! Plain!" It's a new world for Jenna, who doesn't know about rap music, cell phones or thong underwear. But her retro sensibility turns out to be very au courant, especially when she leads a chic Manhattan party in a group line dance of "Thriller," the movie's most delightful set piece.

Even in a movie that's essentially a knockoff of a better original, director Gary Winick ("Tadpole") does a good job of portraying New York as what it often seems: high school with an expense account. The skills Jenna needs to succeed as an adult are immediately available to her because she thinks everyone else is a teenager, too. Which, of course, we all are. In the movie's most touching sequence, Jenna goes home to visit her parents, and her mother instinctively decorates her pancakes with a smiley face made out of blueberries. It makes perfect sense that a chronologically challenged adult would go to the one place where they always treat you like a child, no matter how old you are.

13 Going on 30 (98 minutes, at area theaters) is rated PG-13 for some sexual content and brief drug references.

Mark Ruffalo and Jennifer Garner as the boy next door and the girl whose birthday wish came alarmingly true. Jennifer Garner discovers she can finally leave the Kleenex box alone in "13 Going on 30."