Cady, a sixteen-year-old homeschooled daughter of zoologist parents, returns to the US with her parents after residing in Africa for 12 years. Cady is unprepared for her first day of public high school. With the help of her new friends, Janis and Damien, Cady learns about the various cliques. They warn her to avoid the school's most exclusive clique, the Plastics, the reigning trio of girls led by the “queen bee” Regina George. The Plastics take an interest in Cady, inviting her to sit with them at lunch and go shopping with them after school. Upon realizing that Cady got accepted into the Plastics, Janis hatches a plan to get revenge on Regina, using Cady to infiltrate the high-strung clique.
Having ingratiated herself with the Plastics per Janis' plan, Cady learns about a "Burn Book." This notebook is Regina's top-secret notebook filled with vicious rumors, secrets, and gossip about all the other girls (and teachers) in their class. Cady soon falls in love with Regina's ex-boyfriend Aaron Samuels, whom Regina successfully steals back from Cady in a fit of jealousy. Hurt and frustrated, Cady continues with Janis' plan which involves cutting off Regina's "resources," which include separating her from her boyfriend Aaron, destroying her beauty, and turning Regina's fellow Plastics, insecure rich girl Gretchen Wieners and sweet, but ditzy, Karen Smith against her. Cady starts feigning