After drunken partying the previous evening, university student Theresa "Tree" Gelbman wakes up on her birthday in the dorm room of her classmate Carter Davis. She ignores a phone call from her father David and dismisses Carter while leaving his room and throws away a birthday cupcake given to her by her sorority housemate Lori Spengler. Tree also 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 campus mascot - a baby. Tree immediately wakes up in the same way she had previously - in Carter's bed - and is unnerved to find the previous day's events repeating themselves. Baffled, she nonetheless relives the day and uses this second chance to avoid the tunnel and reach the party. However, the masked killer follows her to the party and murders her again. Tree again wakes up in Carter's bed, realizes that she is in a time loop and barricades herself in her sorority house room to avoid death. The killer, however, is already hiding there and slays her a third time..