Cady is a 16 year old girl who has lived in Africa and been home-schooled all her life. When her parents move to America, they enroll her in a mainstream school, where she meets Janice and Damian. She also, on her second day, meets Regina, Gretchen and Karen - the Plastics.The Plastics are the most popular girls in school, they have a 'Burn Book', in which they write really bitchy things about other girls and even have their own set of 'rules', like : 'Only wear pink on Wednesday', or 'Jeans on Friday'. Janice, Damian and Cady think it would be funny to have Cady become a Plastic for a week, and spy on them.
After Regina, the leader or Queen Bee, kisses the boy Cady has a crush on, Cady, Janice and Damian get revenge on her, by feeding her Kalteen bars, which make you put on weight, giving her foot cream for her face, and using the classic three-way calling method that plenty of girls these day use. Things get so bad, though, that Regina is kicked out of the Plastics, making room for a new Queen Bee. Gretchen and Karen now pick Cady. Cady becomes a Plastic for real, and Regina gets revenge by writing about herself in the 'Burn Book', copying all of the entries, throwing them around the school, then blaming the whole thing on Cady, Gretchen and Karen.
The girls are all ordered into the school hall, where they undergo a 'Trust' workshop, because of this. Janice tells Regina that Cady becoming a Plastic was just a joke, and tells her all of the mean things that Cady did to her. Regina runs out into the street and Cady follows her, where Regina starts shouting at her, when she is hit by a bus. She fractures her spine, and while the Spring Fling is on, Cady is away with the Mathletes at a competition. When she gets to the dance, her name is called as Queen. She apologises for being so cruel to everyone in the 'Burn Book', even though it wasn't her, and breaks up her crown, tossing bits of it to the crowd.
We then flash forward a year, where we see Janice going out with Kevin G, a Mathlete and MC. The Plastics break up, but remain friends, and we see a new group of Plastics walk by, in exactly the same way we saw Regina, Gretchen and Karen walk past at the start. We see how they got on, with Cady turning into a 'normal' person again, Regina playing lacrosse, Gretchen hanging out with new friends and Karen using her 'phsycic' powers to tell the weather.