Photo: Actress Jennifer Garner stars in 13 Going on 30, portrays a girl who plays a game on her 13th birthday and wakes up the next day as a 30-year-old woman. (AP /Virginia Lee Hunter)

Jennifer Garner goes big in her first starring role in a comedy, and it pays off big-time, in 13 Going On 30. The star of TV's Alias plays an awkward 13-year-old girl who yearns for adulthood and is transported to her future as a 30-year-old woman, so comparisons to Big are inevitable. (And there are more than a couple of nods to that 1988 movie: She marvels at Manhattan through the sun roof of a limousine and makes simplistic suggestions at the office that are interpreted as genius.) That's OK, though, because Garner accomplishes something that Tom Hanks did, too: She truly makes you believe you're watching a child trapped inside the body of an adult, and is absolutely charming in the process. It's a joy to see Garner's radiant smile replacing her usual tough-gal scowl, to watch her use her physicality for having fun instead of kicking butt. Even during a potentially cringe-worthy moment at a party, when she busts out the moves from Michael Jackson's Thriller video and everyone else joins in, Garner is so lovably goofy, she makes it palatable. As Jenna Rink, she wishes during her 13th birthday party that she could be "30, flirty and thriving," to quote the fashion magazine that's her bible. That was back in the big-haired days of 1987, which brings me to a truly nitpicky quibble: The music, which is a huge part of the movie, is just a little bit off. Jenna listens obsessively to Jessie's Girl by Rick Springfield, which came out in 1981, and Madonna's Crazy for You, a hit in 1984, plays during a game of Seven Minutes in Heaven. The aforementioned Thriller was big in 1983. Anyway, Jenna suddenly wakes up in a Greenwich Village apartment (with a fabulously stocked closet) and finds she's dating a New York Rangers hockey team star and has a high-powered gig at the same fashion magazine she read as a child. The logic in the script from husband-and-wife Josh Goldsmith and Cathy Yuspa, who also wrote What Women Want, gets shaky here. At times, Jenna seems to have the mentality of a young girl: She gets giggly and grossed out by the idea of making out with her boyfriend, for example. But at other times, she functions capably as an adult: Jenna commandeers a redesign of the magazine and competes with Lucy (Judy Greer), the meanest girl in school who, she's now surprised to learn, has become her best friend and co-worker. Gary Winick, who previously directed the low-budget Tadpole and produced indie films on digital video including Tape and Chelsea Walls, also goes big here, depicting Jenna's adventures with bright colour and high energy. But this teen-girl fantasy has some poignant moments, too, especially with Mark Ruffalo, who plays a grown-up version of the geeky boy next door she jilted as a child. Ruffalo, the co-star of You Can Count on Me who's great in everything he's in, even the self-serious In the Cut, gives the movie some emotional weight and resonance when everyone else is flitting about being girlie. 13 Going On 30 opts for the obviously happy ending when there are at least a couple others that would convey the film's message. But now I'm probably just being too grown-up and uptight.