Mean Girls is a 2004 American teen comedy film directed by [Mark Waters, and written by Tina Fey. The film stars Lindsay Lohan, Rachel McAdams, Tim Meadows, Ana Gasteyer, Amy Poehler and Fey. It is based in part on Rosalind Wiseman's 2002 non-fiction self-help book, Queen Bees and Wannabes, which describes female high school social cliques and the damaging effects they can have on girls. Fey also drew from her own experience at Upper Darby High School as an inspiration for some of the concepts in the film. The film introduced Amanda Seyfried in her film debut.
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", which is led by beautiful and manipulative queen bee Regina George and includes the insecure but rich 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. Cady becomes attracted to Regina's ex-boyfriend, Aaron Samuels, and purposely fails math, a subject she is gifted at, in order to have an excuse to talk to him. Regina finds out about Cady's crush on Aaron and jealously steals him back at a Halloween party by kissing him in front of Cady. This spurs Cady to fully commit to Janis' plan to cut off Regina's "resources": involving breaking Regina and Aaron up, tricking Regina into eating "Swedish 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 the Burn Book all over the school, quickly inciting massive socially motivated brawls throughout the halls. To avoid suspicion, Regina inserts a fake libel of herself in the book in order to blame Cady, Gretchen, and Karen – the only female juniors not mentioned in the book. Karen convinces the school's principal, Ron Duvall, that they did not write the book, who soon quells the fighting and gathers all of the junior girls in the gymnasium. Math teacher Ms. Norbury, whom the Burn Book defamed as a drug dealer, makes the girls face the ways they all treat each other and apologize to each other and the teachers; the plan sees success, as friendships are rekindled. When Janis' turn comes, she defies Norbury, confessing her plan to destroy Regina with Cady's help and openly mocking Regina, drawing praise from other students Regina bullied. Pursued by an apologetic Cady, Regina storms out of the school and is struck by a school bus, breaking her spine, and rumors spread that Cady pushed Regina in front of the bus.
Shunned by her peers and grounded by her parents, Cady takes full blame for the Burn Book. After making amends with Regina, she joins the Mathletes in the state championship finals to make up for the math tests she failed. Cady answers the tiebreaker correctly, and they win the championship for the school. At the Spring Fling dance, Regina's new boyfriend, Shane Oman, is elected King, while Cady is elected Queen. Onstage, Cady declares that all of her classmates are wonderful in their own way, snaps her plastic tiara, and distributes the pieces to other girls in the crowd. She then reconciles with Janis, Damian, and Aaron, and reaches a truce with the Plastics.
The Plastics disband over summer vacation: Regina joins the lacrosse team to deal with her anger, Karen becomes the school weather reporter, and Gretchen joins the "Cool Asians" clique. Aaron graduates from high school and attends Northwestern University while starting a relationship with Cady, who visits him during the weekends. Janis begins dating Mathlete Kevin Gnapoor, whom she initially disliked. As Cady reflects on the societal peace that has taken over North Shore High, a group of new "Junior Plastics" has arisen, and Cady imagines them being hit by a bus like Regina was.