On May 26, 1987, Jenna Rink, a gawky girl, yearns to be popular, but the only way she can get the ruling clique – the “Six Chicks,” led by the arrogant Lucy “Tom-Tom” Wyman – to attend her upcoming 13th birthday party is by doing their homework. Jenna’s best friend, the geeky boy Matty Flamhaff, (with the horrible nickname of Beaver because he looks like the star of “Leave It To Beaver”) arrives early to the party to give her a bright pink, dream dollhouse that he built for her. He sprinkles his second gift, a packet of glittery “magic wishing dust.” on its roof.
The Six Chicks soon show up with the cutest boys in class and make Jenna play “Seven Minutes in Heaven” While Jenna waits, blindfolded, in a dark closet, thinking a popular boy she has a crush on is about to enter, the Six Chicks vanish with all the boys, half the food and Jenna’s completed homework. It’s Matt who walks into the closet, to Jenna’s horror. She locks herself in the closet and cries, wishing to be 30; above her, the glittering wishing dust from the dollhouse gently rains down.
The next morning, Jenna awakens in a gleaming Fifth Avenue apartment. Jenna’s dream has come true: It is now 2004, and Jenna, at first utterly baffled, particularly by the handsome hunk in her shower, realizes she’s magically turned thirty overnight, with no idea of what happened in the intervening seventeen years.
Jenna discovers that she works for Poise, her favorite fashion magazine. Tough-as-nails Lucy is her co-editor and best friend, but the magazine itself is in serious trouble, having been scooped by a rival magazine named Sparkle so often that the editor-in-chief believes someone inside Poise is tipping them off. Jenna, freaking out like the frightened teen she still is, wants only to find Matty. She gets his address and races down to the Village where the now-grown Matt (Mark Ruffalo) is a struggling photographer. To her confusion, he’s distant and cold, and can’t even fill Jenna on much of her missing past, because she became head of the “Six Chicks,” and never spoke to Matt again. She even became Prom Queen – and Lucy, her only friend, is actually the original “Tom-Tom’ after plastic surgery.
While delighting in her freedom and great clothes, Jenna stumbles through a grown-up world, learning enough of life to advise other 13-year-olds whom she actually prefers hang with. But her slowly-emerging past reveals she was nothing like the sweet, shy girl she’d been the day before: this grown-up Jenna stole ideas, refused to speak to her parents, has office sex with the husband of a co-worker. After Jenna overhears her supposed best friend Lucy badmouthing her, in a plan to save the magazine behind her back, she resolves to fix the sins of the past she can’t remember.
She returns to her hometown in New Jersey and weeps in the same basement closet. Her parents find her there, and they hug. She gets back in touch with Matt, gingerly apologizes and hires him to do the photography on her own new plans for Poise, which is a huge break for him. Even though Matt has a fiance in Chicago who’s eager for him to move there, Jenna and Matt begin to fall for each other.
Everyone loves Matt’s photos and Jenna’s new plans to save the magazine, but when Sparkle shows up yet again with this exact material, including Matt’s own photographs, and with Lucy as their new head, Poise folds. Outraged, Jenna confronts Lucy for stealing, but Lucy scornfully tells her that Jenna was the one sabotoging her own magazine all along; Lucy merely found out about it and did the same thing.
Matt, wounded by what he thought was Jenna’s betrayal of him, is getting married the next day. Jenna rushes out to the leafy suburb on his wedding day, hoping to convince Matt that she wasn’t the person she’d seemed to be, that he’d marry her if he could see who she really was, but Matt, already in his tux, says the past can’t come back and hands Jenna her pink home-made dollhouse, which he’d kept all these years. While the wedding begins in the background, Jenna leaves in tears, closing her eyes and clinging to the dollhouse — on which a few bits of wishing dust still remain.
When Jenna opens her eyes, she’s back in 1987, on that same 13th birthday night. This time, when Matt finds her huddled alone in the closet, she kisses him. They run upstairs together, bumping into Tom-Tom on the way, Jenna rips up the homework in Tom-Tom’s hands and Jenna and Matt run out of the house, emerging as a wedding couple on the other side of the door, and, as credits roll, they’re moving into a bright pink house just like the dollhouse that started it all.