Mean Girls

After being homeschooled her entire life and having spent the last twelve years in Africa, 16-year-old Cady Heron begins her first day at North Shore High School. She struggles to make friends and is patronized by the teachers, but befriends outcasts Janis Ian and Damian Leigh, who explain the school's various cliques to her and warn her about the "Plastics", a trio of wealthy mean girls consisting of ruthless queen bee Regina George, insecure gossiper Gretchen Wieners, and bubbly airhead Karen Smith. Regina, Gretchen, and Karen take a shine to Cady and unexpectedly invite her to join the Plastics. Upon realizing this, Janis hatches a plan to infiltrate the group and destroy Regina's reputation.
Cady becomes infatuated with her classmate Aaron Samuels. Karen and Gretchen reveal that Aaron is Regina's ex-boyfriend and thus is off-limits, though Regina claims not to care if Cady and Aaron date. Despite Janis's warnings, Cady comes to enjoy hanging out with the group, including writing insulting remarks about their classmates and teachers in a scrapbook called the "Burn Book." At a Halloween house party, Regina promises to talk to Aaron on Cady's behalf, but instead kisses him in front of her and resumes their relationship. Enraged, Cady fully commits to Janis's plan.
Over the following months, Cady, with Janis and Damian's help, manages to trick Gretchen into divulging secrets, sabotage Regina's relationship with Aaron (and spend time with him by pretending to need math tutoring, despite being mathematically gifted), and cause Regina to gain weight by giving her high-calorie snack bars under the pretense that they are diet food. After violating the Plastics' dress code rules by wearing sweatpants on Monday due to her weight gain, Regina is ousted from the group, as Cady replaces her as "queen bee".
While Cady's parents are out of town, she throws a house party, during which she drunkenly admits to Aaron that she does not actually need math tutoring. He derides her for being as manipulative as Regina. Janis and Damian angrily confront Cady for throwing a party instead of attending the former's art show and declare that she has become as fake as she pretended to be. Meanwhile, Regina discovers the truth about the snack bars and inserts slander of herself into the Burn Book, hoping to frame Cady and the other Plastics.
Regina spreads the book's pages throughout the hallways, inciting chaos. Principal Duvall and math teacher Ms. Norbury gather the female junior students in the gym to talk through their social issues. During this, Janis publicly admits her plan to destroy Regina and openly mocks her, much to the other students' excitement. Regina furiously storms out, pursued by an apologetic Cady. While ranting at Cady, Regina is hit by a school bus, fracturing her spine. Rumors circulate that Cady intentionally pushed Regina in front of the bus, resulting her in becoming a social pariah and grounded by her parents.
After realizing a comment she wrote in the Burn Book has framed Ms. Norbury as a drug dealer, causing police to visit the school and interrogate her math class, Cady decides to take full responsibility for the book. Regina, Gretchen, and Karen are spared punishment. To earn extra credit, Cady joins the Mathletes, moderated by Ms. Norbury and led by fellow student Kevin Gnapoor. At the state finals, she correctly answers the tiebreaker question, winning the championship. The team arrives at the Spring Fling dance, where Cady is elected queen, to her surprise. She breaks her tiara and distributes the pieces to others in the crowd, making peace with Janis, Damian, and the Plastics. She later shares a slow dance and kiss with Aaron.
The Plastics disband by the time senior year begins. A fully recovered Regina joins the school's lacrosse team to channel her anger positively, Gretchen joins the "Cool Asians" clique, and Karen becomes the school's weather girl. Cady begins dating Aaron and continues to hang out with Janis and Damian, the former of whom starts dating Kevin. While reflecting on the relative social peace that has taken over the school, Cady notices a new popular clique in the freshman class and wonders how long the "Junior Plastics" will last, imagining them being hit by a school bus.