After a night of drunken partying, university student TREE 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.
Tree immediately wakes up back in Carter's bed, and is unnerved to find the previous day's events repeating themselves. Baffled, she relives the day, this time avoiding the tunnel and reaching the party. However, the masked killer follows her and murders her again. Tree again wakes up in Carter's bed, realizes she is in a time loop and barricades herself in her room to avoid death. The killer, already hiding inside, murders her a third time.
Waking up, Tree explains the events to Carter. He suggests taking advantage of the loop to identify her killer. She spends the next several iterations trailing people she considers suspects but is killed each time. After waking from a loop where she is bludgeoned, Tree faints and awakens in the campus hospital. Her body shows evidence of recovery from multiple traumatic injuries, indicating she has retained physical damage from her previous deaths. When the killer shows up, Tree escapes the hospital in Gregory's car, only to be pursued and killed again.
Back in Carter's bed, Tree convinces him of her predicament by demonstrating her knowledge of the day's events. Tree admits to harboring tremendous self-loathing, particularly from pushing away her father after the death of her mother three years ago. Tree sees a local news report on John Tombs, a serial killer being held at the campus hospital. Concluding that Tombs is her killer, Tree rushes to the hospital to warn of his escape. Tombs breaks free and nearly kills Tree, but Carter follows and rescues her. Tombs kills Carter before chasing Tree to a nearby bell tower, where she subdues him with a crowbar. Before delivering a fatal blow, she realizes that if she kills Tombs and ends the loop, Carter will remain dead forever. She runs to the top of the tower and hangs herself.
Tree awakens in Carter's dorm room and finds him alive again. Now confident in solving her murder, she proceeds happily through the day. She ends her affair with Dr. Butler and meets her father for lunch, where the two begin to reconcile. That night, she goes to the hospital and traps and kills Tombs. Relieved to finally be free, she celebrates her birthday in Carter's room and eats the cupcake Lori gave her.
Tree wakes up still in the loop. Horrified, she returns to her room with the intent to run away, where Lori offers her the cupcake again. Tree realizes the previous loop was the only time she had ever eaten the cupcake, and she had died in her sleep. Tree realizes Lori is her true killer. Lori had poisoned the cupcake, but when Tree failed to eat it, Lori used her job as a nurse at the hospital to frame Tombs for Tree's murder. Tree threatens to take the cupcake to the police, but Lori attacks her. Lori admits to also having an affair with Dr. Butler, whose preference for Tree drove Lori mad with jealousy. In the ensuing fight, Tree stuffs the poisoned cupcake in Lori's mouth, then kicks her out a second-story window to her death.
At a restaurant, Tree and Carter muse over the day's events and he offers her his room for the night. The next day, Tree wakes up believing she is still in the time loop, but Carter quickly reveals he was just playing a prank on her, and it is really the next day. Tree is too relieved to be angry at him, and the two kiss.
