Cady has been home-schooled for her first 16 years, so when she enters a public school for the first time, the movie introduces her (and us) to the complicated interactions of adolescent girls. She initially bonds with two social outcasts, who devise a plan for Cady to infiltrate the Plastics, a trio of popular girls led by the vindictive Regina.
After she gains acceptance in the clique, Cady begins to subtly undermine Regina by trying to make her gain weight and turning the other Plastics against her in essence, the bullied becomes a bully. As she is more and more successful, Cady loses her sense of self and morphs into a new version of the queen bee. What started as a joke becomes real as she turns just as spiteful and mean as Regina.
Mean Girls brings shades of gray to the typical bully/victim paradigm. Everyone here is a player in an endless cycle of bullying and being bullied. When Cady succumbs to the pettiness and vanity of the Plastics, the movie shows how intoxicating popularity can be, and how easy it is to switch from victim to bully, and back again.