Plot

Anna Coleman (Lindsay Lohan) is an average teenage rebel whose constant victims of her nature are her stodgy mother Tess (Jamie Lee Curtis) and annoying younger brother Harry (Ryan Malgarini). Sources of irritation include a rock band Anna is in, which Tess dislikes, and Tess's upcoming wedding to her boyfriend Ryan (Mark Harmon), which Anna is emotionally not ready for due to her father's death three years ago. Also contributing to Anna's irritation is her arch-rival, Stacey Hinkhouse, who never seems to stop torturing her and has convinced Tess that she and Anna are still best friends (which was formerly), and her English teacher, Mr. Elton Bates, who gives her an "F" on every assignment, no matter how hard she tries.

When the entire family eat out in a Chinese restaurant, Anna and Tess quickly start fighting again: Anna wishes to participate with the rest of her band in a band audition, however the show is the same night as Tess's wedding rehearsal. Hearing the argument, Pei-Pei's mother meddles in by offering Tess and Anna both fortune cookies. Upon opening them, there is a short earthquake which only they can feel.

The next day, after the stroke of midnight, Tess wakes up and somehow discovers that she and Anna have swapped in each other's body. Confused and unable to work out on how to swap back on their own, they decide to go back to the restaurant to find out what happened. Since Anna has an important test and Tess must go to work, the two are forced into each other's roles. At school, Tess not only realizes Anna was right about Stacey, but is also given a bad grade from Mr. Bates even though she got all the answers correct, and realizes that he was her former classmate, whom she had turned down when he had asked her to the School Prom, and he is taking it out on Anna. Tess confronts Anna's teacher over this in front of Anna's friends, humiliating him and warning him to stop his abusive treatment of her or he will be reported to the school board.

After work with Anna primarily forced to feign her way through most of Tess's appointments apart from a case involving a mother's concerns over her daughter, Anna gives Tess's body a makeover, including new clothes, a new haircut, and an ear piercing. Then, the two go to the restaurant again and speak to Pei-Pei, the daughter of the woman that gave them the fortune cookies. Furious at her mother's meddling, but unable to directly help them, Pei-Pei advises them to read the fortunes in the cookies, as when the fortunes come true, they will swap back.

In the afternoon, Anna attends Harry's parent-teacher conference, where she reads a composition, he wrote about how much Harry admires Anna, but provoked fights so that she would pay attention to him. When Tess takes Anna's test, Stacey writes a note and places it on her desk for her to read, but then makes it look like she was cheating, landing her in detention. Tess is able to finish the test later with the help of Jake, an older student that Anna had a crush on. She also gets revenge on Stacey by erasing all of the answers she had on her test paper and writing "I'M STUPID!" on it.

Meanwhile, Ryan surprises Anna with an interview on a talk show to discuss her new psychology book. She is unable to discuss the meaning of the book, which she has not read, so she improvises by turning the show into a wild romp while angering Tess, who sees the interview on TV, in the process while in the school's teacher's lounge. Afterwards, Anna sees Jake at a coffee shop and bonds with him over similar musical interests. Jake then begins to fall for Tess, when he notices all the characteristics he likes about Anna.

At the wedding rehearsal that evening, Tess and Anna, while still in each other's bodies, read the fortunes, stating that "What you see is what you lack, then selfless love will change you back," but leaves them overly confused. Anna's band-mates come to try to convince Tess to go to the audition, but ultimately decides to use force. However, the band-mates are caught by security, but Ryan surprises Tess and Anna when he gives her permission to go. He also tells Anna that he wanted her to accept him into the family on her own and that he wants Anna to go watch Tess. Seeing Ryan in a new light, Anna leaves to watch her band perform.

At the audition, Tess is unable to play the guitar solo, so Anna unplugs it and plays another guitar backstage while Tess mimes along. The band does a great job and for the first time, Tess realizes how much fun Anna has playing in her band. Back at the wedding rehearsal, Tess asks Anna to have Ryan postpone the wedding, so that Anna will not have to go through marrying him in her mother's body.

Instead, Anna proposes a toast where she finally accepts Ryan by realizing how happy he makes Tess. There is a second earthquake that the whole crowd feels, plus Tess and Anna switch back into their own bodies. On the wedding day, Tess and Ryan marry and Anna and Jake meet and while dancing they share a kiss. By the end of the film, Anna shares a song named "Ultimate."

Harry and Grandpa have an argument and Pei-Pei attacks and wrestles her mother to the ground as she attempts to give the pair the fortune cookies, preventing another terrible mix-up