After a night of drunken partying, university student Theresa “Tree” Gelbman wakes up on her birthday in the dorm room of classmate Carter Davis. She ignores a phone call from her father and dismisses Carter, returning to her room. Her sorority housemate Lori Spengler gives her a cupcake, which she throws away. Tree meets with her married professor, Gregory Butler, with whom she is having an affair. That night, on her way to a party, Tree is lured into a tunnel and murdered by a figure wearing a mask of the school mascot.