13 Going on 30

“I want to look like THOSE people,” 13-year-old Jenna Rink says, pointing to the pictures in a fashion magazine called Poise.

“Oh, those aren’t people, honey,” her mother replies. “Those are models.”

Ah, wisdom. Thus begins “13 Going on 30,” a sweet, unassuming little comedy about girl power and self-esteem that has the impish Jennifer Garner to lead the way.

We start in 1987. Jenna (played at 13 by Christa B. Allen) is an average, self-doubting girl whose best friend is a tubby boy named Matt (Sean Marquette). She wishes her best friends were the Six Chicks, a band of pretty girls led by uber-snob Lucy (Alexandra Kyle). They have the traditional arrangement: Jenna does their homework for them, and they kind of pretend to sort of be nice to her sometimes.

After a particularly humiliating run-in with the Six Chicks at her 13th birthday party, Jenna wishes with great fervor that she were 30; she chooses that age because of the Poise article that talked about being “thirty, flirty and thriving.” She wakes up the next morning to find herself fast-forwarded 17 years to 2004, where she’s a successful 30-year-old editor of Poise magazine, and best friends with the once-envied Lucy (Judy Greer). She has no memory of the intervening 17 years, and her friends and co-workers wonder why all of a sudden Jenna is acting like a 13-year-old. Somehow, she has skipped ahead.

She’s played by Jennifer Garner now, of course, and Garner’s youthful enthusiasm in the role is absolutely contagious. She has young features — accompanied by an angular face and square, mannish shoulders, but that’s beside the point — and she bubbles with the zeal of a girl who has suddenly found that all her dreams have come true.

Jenna looks up Matt (Mark Ruffalo) and is alarmed to learn they drifted away shortly after that fateful birthday party. What’s more, he’s engaged, though perhaps you saw that coming.

More unsettling realizations await Jenna, but I will only hint at them here. Basically, she must discover HOW, exactly, her dreams all came true, and what sort of person she became in the process.

I mention the introspection because it’s wholly unusual for a film based on a fantasy premise to even suggest that a character look within herself, much less for her to be shocked by the results. Most films would let Jenna become princess of the world, give her a mild complication with Matt, let them overcome it, and send them on their merry way. This one, directed by Gary Winick (“Tadpole”), sends us through much more interesting terrain on our way to the happy ending.

I like the scene where Jenna’s magazine is hosting a party, which is turning out to be a dud. Still mired in 1987, she gets the DJ to spin Michael Jackson’s “Thriller,” and soon everyone is dancing. Not only is this one of filmdom’s few spontaneous dance sequences that actually make sense — it’s entirely plausible that a group of 30-somethings would, in fact, know the dance moves from the “Thriller” video — but it has the added humor of underscoring Jenna’s 1987 innocence in a 2004 world. She has no idea, after all, that Michael Jackson’s music is played solely for camp value nowadays.

(That scene also gives us a chance to see Andy Serkis — Gollum himself — moonwalk as Jenna’s boss.)

Written by “What Women Want’s” Cathy Yuspa and Josh Goldsmith, with polishing by Niels Mueller (“Tadpole”), the story uses both the kid-in-a-grown-up’s-body premise and the person-from-the-past-thrust-into-the-present premise without overdoing either of them. Jenna’s confusion in the modern world is limited mostly to being frightened by cell phones, and I was grateful not to see a crazy shopping spree once Jenna realized she was an adult now. (We’ve all seen “Big”; we get it.)

The trouble with it all is that it runs out of steam by the end, and introduces an unnecessary subplot involving a Lucy-sponsored backstabbing to boot. Garner and the goofily likable Ruffalo’s chemistry is satisfactory, but it can’t hold the movie together when it’s collapsing on itself. That said, it’s a pleasant surprise that the movie holds up as well as it does for as long as it does. Jennifer Garner might have a future as a big-screen star.

B- (1 hr., 36 min.; PG-13, a little mild profanity, some crude language; should have been PG.)