Jennifer Garner's chameleonic acting abilities have given veracity and weight to the outlandish goings-on in "Alias," the espionage drama that made her a hot commodity. In the C-grade, three-ring superhero circus of "Daredevil," she acted circles around Ben Affleck and added surprising emotional weight to her kung-fu butt-kicking performance. In just one scene as a come-hither high-priced hooker playing hard to get in "Catch Me If You Can," Garner made herself as memorable as Leonardo DiCaprio and Tom Hanks.

And now in "13 Going On 30," Garner follows Hanks' career template, proving herself a potential superstar in a magnetically ingenuous performance as Jenna Rink, a frustrated late-'80s 'tweenager who makes a careless wish around some "wishing dust" and wakes up 17 years later in the va-va-voom body and the chaotic career-girl life of herself at age 29.

Yes, it's basically "Big" with boobs (a physical attribute that has pubescent-on-the-inside Jenna Rink just tickled pink) -- and like its opposite-sex predecessor, this movie is fueled by the irresistible exuberance of its star's inner child.

While witty and perfectly fit to Garner's talents as a comedienne (the girl can take a pratfall like nobody's business), the script, isn't particularly creative (Wishing dust? That's the best they could do?) as it follows Jenna through discovering a hunky towel-clad fella in her apartment ("Eewww!"), learning that she's grown up to be an editor at her favorite fashion magazine, and realizing that it's not all she'd hoped it would be.

But Garner positively shines. Whether she's chomping on chewy Fruit Roll-Ups wrapped absentmindedly around her finger during an emergency editorial meeting, cracking up uncontrollably when the hunk tries to put a tongue in her ear, or visiting her parents and crawling into bed with them for midnight comfort, the actress never has a false moment in the entire movie. She is 100 percent 13 years old -- even as she adapts quickly to modern conventions (director Gary Winick doesn't over-milk gags like her confusion over those strange ringing sounds coming from her purse) and even as she begins to figure out that the adult Jenna she's jumped into has been a shallow, conniving, back-stabbing cat.

Desperate for a friendly face in the resulting self-doubt, she tracks down Matt (Mark Ruffalo), young Jenna's awkward, earnest best friend, who is more than a little surprised to see her since the girl he knew hasn't talked to him since she started running with the "cool" crowd in junior high school. He's thrown for quite a loop because seeing Jenna again -- especially behaving the way she did when he had a teenage crush on her -- has rekindled feelings that threaten his recent engagement to another woman.

While considerably more understated, Ruffalo matches Garner's knack for tapping into the psychological remnants of childhood, letting slip the still-tentative part of Mark's personality and the buried kid-ish giddishness that emerges when he lets down his guard.

The plot's forward momentum is predicated on some pretty contrived elements, like the fact that the magazine (run by the very funny Andy Serkis -- Gollum in "The Lord of the Rings" films) is in dire need of revitalization and Jenna's child-like brainstorms are supposedly just the breath of fresh air to do the trick. (In real life her ideas would never fly.)

Director Winick ("Tadpole") is a little too reliant on peppy music video sequences (the narrative crutch of far too many upbeat chick flicks), and the notion that a 13-year-old girl would make a wish to be 30 is a little hard to swallow in the first place. Twenty-five? Sure. But 30 is the official threshold of becoming "old" when you're that age.

The movie also cops out of some awkward moments -- as when Winick skips, without explanation, to the office the next morning just after the hunky boyfriend tries to seduce Jenna with a striptease. ("Oh, gross!")

But the picture's every negative is balanced out by Garner's perfectly bewildered innocence and comedic acumen. Without her, "13 Going on 30" just wouldn't have the same charm. With her, it's magic.