Sixteen-year-old homeschooled Cady Heron and her zoologist parents, Betsy and Chip Heron, return to the United States after a twelve-year research trip in Africa, settling in Evanston, Illinois. On her first day ever of attending a school, North Shore High School, Cady attempts to make new friends, but to no avail. The next day, she meets and befriends Janis Ian and Damian Leigh. They educate Cady on the school's various cliques and warn her to avoid the most popular and infamous one, the "Plastics": beautiful and manipulative queen bee Regina George, rich but insecure Gretchen Wieners, and sweet but dimwitted Karen Smith. The Plastics take an interest in Cady after defending her against a sexist classmate, and invite her to sit with them at lunch. After learning of the invitation, Janis asks Cady to befriend them and to spy on them for her.
Cady soon learns about the "Burn Book", a scrapbook the Plastics have made that is filled with horrible rumors, secrets, and insults about other girls and some teachers at school. Using the book, Janis devises a plan to get back at Regina but Cady is reluctant, thinking Regina is a good friend.
