15-year-old Anna Coleman (LINDSAY LOHAN) is a typical teenager who likes hanging out with her friends, is constantly annoyed by her younger brother, Harry (RYAN MALGARINI), and thinks that her mom, Tess (JAMIE LEE CURTIS), is a square and out of touch with her times.
Tess, on the other hand, is a harried psychologist and widow whose wedding to Ryan (MARK HARMON) is fast approaching. She's concerned not only with Anna's less than enthusiastic response to that, but also with her growing rebellious attitude.
That comes to a head in a Chinese restaurant over Anna wanting to play with her garage band in an important tryout that just so happens to fall on the night of the rehearsal dinner. An older Asian restaurant worker then brings them fortune cookies whose duplicate fortunes put a spell on both women.
The next morning, Anna wakes up Tess' body and vice-versa, much to the shock and dismay of both of them. Not wanting to let anyone know what's happened, the two try to pose as each other, with Anna (in Tess' body) heading off to work and dealing with the thought of marrying Ryan the next day, while Tess (in Anna's body) heads off to school where she meets Jake (CHAD MICHAEL MURRAY), the hunky young guy Anna likes.
With the band gig and the rehearsal dinner fast approaching, Anna and Tess try to fool everyone else into believing who they are, all while learning a thing or two about how the other actually lives.