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.
College student Ryan wakes up in his car on Tuesday, September 19. Returning to his dorm room, he walks in on his roommate Carter and Carter's girlfriend Tree. He resumes work on an experimental quantum reactor with fellow students Samar and Dre. After Bronson, the school dean, shuts down the project for triggering several power outages, Ryan is murdered by someone dressed as Babyface, and wakes up again on Tuesday the 19th. Tree explains her experience Happy Death Day#Plot, and she and Carter agree to help Ryan. They learn the reactor was responsible for creating the loop. The new Babyface tracks Ryan down, but Tree subdues and unmasks him to reveal another Ryan. The second Ryan warns that the original must die for the loop to close. Terrified, Ryan activates the reactor, releasing a powerful energy pulse that knocks everyone unconscious.