As the book is nonfiction, Fey wrote the plot from scratch, borrowing elements from her own high school experience.
The real Janis Ian was one of the musical guests on the first Saturday Night Live episode, in which she sang the song "At Seventeen", which can be heard playing in the background when the girls are fighting at Regina's house.
Cady soon learns about Regina's journal "The Burn Book", which is filled with rumors, secrets, gossip, and insults about the other girls and some teachers at school.
Despite this discovery, Cady decides to forego Janis's scheme for ethical reasons.
Other characters bullying Caplan's character persistently call her a lesbian throughout the film; the real Janis Ian is an out lesbian.
Lindsay Lohan first read for Regina George, but the casting team felt she was closer to what they were looking for in the actress who played Cady, and since Lohan feared the "mean girl" role would harm her reputation, she agreed to play the lead.
Michaels contacted Paramount Pictures, who purchased the rights to the book.
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 reconciles with Janis, Damian, and Aaron, and reaches a truce with the Plastics.
By the start of the new school year, the Plastics have disbanded.
Regina has joined the lacrosse team to deal with her anger, Karen has become the school weather reporter (claiming that her breasts can tell when it is raining), and Gretchen joins the "Cool Asians".