Raised in the African bush country by her zoologist parents, Cady Heron thinks she understands "survival of the fittest." But the law of the jungle takes on a whole new meaning when the home-schooled 15-year-old enters public high school for the first time and must survive the schools mean girls.
Originally, Amanda Seyfried was supposed to play Cady, and Lindsay Lohan was supposed to play Regina.
Cady Heron (pronounced Catty) has spent her first 15 years living in the African jungle with her Zoologist parents. However, when she moves out of , she has to learn the rules of high school, a jungle in itself. She instantly makes friends with two outcasts, Damian and Janis. They tell her about the “Cliques” of High School. Soon Cady meets the Plastics, High School Royalty, the three most beautiful and popular girls, consisting of Regina, the unofficial leader, Gretchen, Regina's full-time follower, and Karen, "one of the dumbest people you will ever meet." After a chance encounter they let her into their group, but Cady, wanting to keep Janis and Damian as friends, is unsure. The two convince Cady to keep her relationship with the Plastics, only so that they can know their dark secrets. Janis was once friends with in middle school and wants to get back at her. However, events turn for the worse when Cady falls for 's ex-boyfriend, Aaron Samuels. When finds out, she seeks revenge on Cady by taking Aaron back. When Cady finds out, what started out as a game to discover secrets turns into a plan to destroy . Although, the more time Cady spends more time with the Plastics, she, too, begins to become one and starts to lose Damian and Janis as friends.