THE MEAN GIRLS Cady Heron, the 16-year-old homeschooled daughter of zoologist parents, recently moved from Africa, is unprepared for her first day of public high school at North Shore High School in Evanston, Illinois. With the help of social outcasts Janis and Damien, Cady learns about the various cliques. She is warned to avoid the school’s most exclusive clique, the Plastics, the reigning trio of girls led by the acid-tongued queen bee Regina George. Regina was once Janis’s best friend, but they have grown to despise each other since in 8th grade when Regina started a rumor that Janis was a lesbian.
However, the Plastics take a shine to Cady and invite her to sit with them at lunch and go shopping with them after school. Upon realizing that Cady has been accepted into the Plastics, Janis hatches a plan to get revenge on Regina for what she did to her, using Cady as a pawn in order to infiltrate the Plastics. Having ingratiated herself with the Plastics per Janis’s plan, Cady learns about the “Burn Book,” a top secret notebook of Regina’s filled with slandering rumors, secrets, and gossip about all the other girls 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 during a Halloween party. Cady, who now hates Regina, goes ahead 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 against her: insecure rich girl Gretchen Wieners and sweet but dimwitted Karen Smith. Cady then starts pretending failure at math to get Aaron’s attention and soon choreographs Aaron’s breakup with Regina by confessing Regina’s infidelity with another boy.
Cady also convinces Regina to eat high-calorie nutrition bars by claiming that they actually help one lose weight. She also turns Gretchen against Regina by making her think Regina thinks of Cady as a better friend instead of her. In her efforts to get revenge on Regina, Cady gradually loses her individual personality and remakes herself in the image of Regina. Her act soon becomes reality, and she becomes as spiteful as Regina, abandoning Janis and Damien in the process and focusing more on her image. Regina, now slightly overweight due to Cady’s diet sabotage, is excluded from the Plastics and Cady becomes the new Queen Bee.
In celebration of her newfound status, Cady throws a party with the Plastics and does not invite Janis or Damien. Janis and Damien then renounce Cady as a friend. During the party, she also alienates Aaron with her unsavory new personality. Regina discovers the truth about the bars she has been eating and strikes back by spreading the entire contents of her Burn Book around the school, inciting a riot; to avoid suspicion, Regina also inserts fake slander of herself in the book, in order to focus blame on Cady, Gretchen, and Karen. The riot is eventually quelled by Principal Duvall.
Math teacher Ms. Norbury-who also appeared in the Burn Book, where Cady slandered her by saying that she sold drugs—makes the girls realize that all of them are guilty of hurting their peers. She has each girl confess and apologize to the rest of the girls. Janis confesses her plan to destroy Regina with Cady’s help, and openly mocks Regina with the support of the entire school. Regina storms out, pursued by an apologetic Cady, and gets hit by a school bus in her haste; rumors circulate that Cady intentionally pushed Regina in front of the bus.
Now without friends, shunned by Aaron, and distrusted by everyone at school, Cady decides to make amends by taking full blame for the Burn Book. Though severely punished by her confession, her guilt dissolves and she returns to her old personality. As punishment for her part, Ms. Norbury has Cady join the Mathletes—which Damien and Regina both described earlier as “social suicide”—in their competition. There, Cady has an epiphany competing against a very unattractive girl, realizing that even if she made fun of the girl’s appearance, it would not stop the girl from beating her.
Cady wins the tournament and returns to school for the Spring Fling. At the Spring Fling dance, Cady is elected Spring Fling Queen and gives a speech to her class that her victory is meaningless; they are all wonderful in their own way and thus the victory belongs to everyone. As a symbolic gesture, she breaks her tiara and distributes the pieces to her fellow classmates. Cady makes up with Janis and Damien, reconciles with Aaron, and reaches a truce with the Plastics.