Sixteen-year-old homeschooled Cady Heron (Lindsay Lohan) and her zoologist parents (Ana Gasteyer and Neil Flynn) return to the United States after a research trip in Africa, settling down in Evanston, Illinois. Attending a public school for the first time, Cady meets new classmates Janis (Lizzy Caplan) and Damian (Daniel Franzese). The two warn Cady to avoid the school's most popular clique, the "Plastics", who are led by queen bee Regina George (Rachel McAdams) and include rich but insecure Gretchen Wieners (Lacey Chabert) and sweet airhead Karen Smith (Amanda Seyfried). The Plastics take an interest in Cady, and invite her to sit with them at lunch. Seeing that Cady is getting along with the Plastics, Janis hatches a plan of revenge against Regina for some past slight, using Cady as the infiltrator.
Cady soon learns about Regina's "Burn Book", a journal filled with rumors, secrets, and gossip about the other girls and some teachers at school. Despite this discovery, Cady decides to forgo Janis's scheme for ethical reasons. Meanwhile, Cady begins to have a crush on Regina's ex-boyfriend, Aaron Samuels (Jonathan Bennett), whom a jealous Regina proceeds to steal back at a Halloween party in front of Cady. This spurs Cady to fully commit to Janis's plan to cut off Regina's "resources", which involve breaking Regina and Aaron up, tricking her into eating nutrition bars that actually make her gain weight, and turning Regina's fellow Plastics against her. In the process, Cady unwittingly remakes herself in Regina's image, becoming spiteful and superficial, and abandons Janis and Damian.
When Regina is finally made aware of Cady's treachery, she retaliates by spreading the contents of her Burn Book all over the school, quickly inciting a riot. To avoid suspicion, Regina inserts a fake libel of herself in the book in order to blame Cady, Gretchen, and Karen, the only female students not mentioned in the book. Principal Duvall (Tim Meadows) soon quells the riot, and gathers all the girls in the school in the gymnasium. Math teacher Ms. Norbury (Tina Fey), whom the Burn Book defamed as a drug dealer, makes the girls face the terrible way they all treat each other, confess their transgressions, and apologize to each other and the teachers. When Janis's turn comes, she confesses her plan to destroy Regina with Cady's help, and openly mocks Regina with the support of the entire school. Pursued by an apologetic Cady, Regina storms out and gets hit by a school bus, breaking her spine.
Without any friends, shunned by Aaron, and grounded by her parents, Cady takes full blame for the Burn Book. After making amends with Regina, Cady's guilt soon dissolves and she returns to her old personality. As part of her punishment for lying and failing Norbury's class, she joins the Mathletes in the state championship finals, and ends up winning the competition for her team after her opponent answers incorrectly. At the Spring Fling dance, Regina's new boyfriend, Shane Oman, is elected King, while Cady is elected Queen. Onstage, Cady declares that all her classmates are wonderful in their own way, breaks her plastic tiara, and distributes the pieces to some other girls. She then makes amends with Janis and Damian, reconciles with Aaron, and reaches a truce with the Plastics, who then disband.
By the start of the new school year, Regina has joined the lacrosse team to deal with her anger, Karen has become the school weather reporter (claiming earlier that her breasts can tell when it is raining), and Gretchen joins the "Cool Asians". Aaron graduates from high school and attends Northwestern University, Janis begins dating Kevin Gnapoor, whom she initially despised, and Cady declares that she is now normal. A group of new "Junior Plastics" has arisen, and Cady imagines them being hit by a bus.
Who inserts a fake libel of herself in the book.
Regina