NOTE: This spoiler was sent in by Skipper who says... "This movie was pretty much an average chick-flick...the events might be slightly out of order"
The movie opens with 13-year-old Jenna Rink having her pictures taken. The photographer takes the picture when she's not ready, and as a result her school picture is not exactly cover model material. She's walking down the hall with her best friend, Matt, when a popular girl named Tom-Tom and her clique, the "Six Chicks" approach her. Tom-Tom tells Jenna that she can't come to her birthday party because she and her friends have to work on a project. Jenna volunteers to do the assignment for them and they agree to come to her party. Next we see Jenna and Matt walking home and Jenna talking about how she wants to be a Six Chick. Matt says it's mathematically impossible for there to be seven Six Chicks. We see that they are neighbors and go into the houses next door to each other.
Next we see Jenna before her party, stuffing her bra when her parents enter her room with a video camera. Her mom takes out the padding and they have a conversation. Jenna says she wants to be 30 and flirty, like the magazine article on her bed, but her mom says she should be original. Matt comes over and gives Jenna his gift: a Jenna doll-sized dream house that he worked on for three weeks, and some "wishing dust." He opens the packet of wishing dust and scatters it over the dream house.
Tom-Tom and her friends show up with some popular boys in tow, and Jenna hastily shoves the dream house into the closet. Matt is dancing around when they arrive, and Jenna is obviously embarassed by him. She rudely tells him to do whatever he wants, and he goes to his house to get his electronic piano. Tom-Tom suggests that they play 7 Minutes in Heaven. She blindfolds Jenna, asks where the project proposal is, and sticks Jenna in the closet. She grabs the project proposal and her friends take some food, and they're on their way out when Matt comes back. Tom-Tom tells him that Jenna is in the closet waiting for him. He goes in and Jenna, thinking that he's one of the popular boys, says, "I've been waiting for you, Chris." Matt reveals himself and Jenna gets angry. She kicks him out and stays in the closet, wishing to be 30. Some of the wishdust spills on her.
Jenna wakes up in a 30-year-old's body (Jennifer Garner) and takes a few minutes to freak out about the situation.
She goes through her mail and sees some bills and a letter that says Sparkle on it with her name on it (important later). A guy comes out of the shower and takes off his towel, which thoroughly freaks her out and she grabs a coat and shoes and runs out the door. Downstairs at the curb, a lady named Lucy (Judy Greer) tells her to get in the car. Jenna hesitantly does, and the two are off to work at Poise magazine. At work, their boss (Andy Serkis) tells them that Sparkle magazine has had basically the same stories and covers as them for the past few months, and it is selling much better. Apparently someone from Poise is leaking information to Sparkle. Jenna asks her assistant to track down Matt and goes to visit him. Her assistant also tells her that Jenna stopped taking calls from her parents.
At Matt's (Mark Ruffalo) apartment, he hardly recognizes her and tells her that they're not friends and he hasn't seen her since high school. They go back to Jenna's apartment. She is informed that after the closet incident, she threw the dream house at Matt and they were never friends from that day on; instead she became the leader of the Six Chicks and prom queen in high school. He says he thought she was still friends with Tom-Tom, and she realizes that Lucy is Tom-Tom grown up. Jenna gets a call from a limo company confirming service to a company party that evening. Jenna invites Matt before he leaves. On her way to the party, Jenna is in the elevator with a 13-year-old girl who lives near her. She introduces herself and the girl says that she never bothered to talk to her before even though they're neighbors.
At the party, the editor of Sparkle shows up and makes nasty remarks about Poise. People start to leave early and Jenna suggests putting on better music. She goes to the DJ and puts on Thriller, and starts dancing to it. Matt shows up and after some persuasion from Jenna, starts doing the Thriller dance with her and people gradually join in. At work, the boss says that Sparkle is outselling them so much that Poise had to redesign. Later that evening Jenna and Lucy are at a restaurant getting drinks and Jenna runs into Matt and his fiance. She also meets up with her boyfriend, the guy in the shower from before. It turns out that he's a hockey player. They go back to his apartment and he proceeds to do a striptease for Jenna, which grosses her out and she leaves.
We see her at work the next day and her secretary is giving her messages. People left her all sorts of nasty messages. The husband of one of Jenna's co-workers comes into her office and starts kissing her. She pulls away and reminds him that he's married and the guy says that didn't seem to bother her last time. Jenna realizes what sort of person she has become and storms out. On her way out, she overhears Lucy talking to someone else about how Jenna took someone's ideas and then fired them and how Lucy plans to go solo on the redesigning project instead of working with Jenna as usual. Jenna goes to visit Matt and tells him that she's not the same person as he thinks she is. After some conversing, Jenna goes back to her parents' house. Her parents come home from their vacation in the Bahamas to find Jenna sitting in the closet. She stays over there and in the morning asks her mom what she would go back and do differently if she could. Her mom replies she wouldn't correct any mistakes she had made because without mistakes, she wouldn't have learned how to make things right.
Jenna decides to hire Matt, who is a photographer, to help with her redesign project. They take pictures of high school students, students at the winter formal, etc. Jenna's theme for the redesign is Class of 2004. Jenna and Matt spend more time together because of the project. One evening they go to a park and go on the swings and end up kissing.
The movie cuts to Jenna telling a bunch of 13-year-old girls at a sleepover about it and they swap tough love stories and dance around the room.
Jenna presents her redesign theme to her boss at a meeting and explains that it's pictures of ordinary people who everyone knows, like their friend's older sister, rather than some overly glamorous model who nobody can identify with, like in Lucy's theme. Her boss loves the idea and Lucy is unhappy about it. She goes into Jenna's office one day, opens a drawer, and finds letters from Sparkle addressed to Jenna; it turns out that she was the one leaking the information. She then calls Sparkle magazine, and we cut to Matt, who has come to visit Jenna. He finds Lucy in her office and Lucy tells him that Jenna decided not to use his photos. She asks him to sign a release form in case they want to use the photos in a catalog. Jenna comes back to the office to find out that Lucy pitched the Class of 2004 idea to Sparkle and had been made their chief executive. She was able to use the photos because Matt had signed the release form. Jenna goes to Matt's apartment to straighten things out, but his fiance comes to the door instead and she tells Jenna that the wedding is the following day.
Jenna decides to go to the wedding at the last minute and arrives right before the wedding starts. She rushes to see Matt and he admits he had feelings for her but he won't let down his fiance and says that sometimes you get everything you want, but not the dream house. He then takes the dream house out of his closet and Jenna asks if she can keep it. She tells him that she loves him before she leaves and he says that he's always loved her. Then she sits down in the yard with the dream house and cries as she hears the wedding starting. There is still some wishing dust on the dream house, and the wind blows it over her.
Next thing we know, Jenna is back in the closet as a 13-year-old. This time, she removes the blindfold and when Matt comes in she kisses him and then tells Tom-Tom she doesn't want to be like her and pretty much kicks her out.
Then the movie cuts to Jenna and Matt getting married and moving into their dream house, and the movie ends with them sitting on the a couch on the front lawn of their pink dream house.