Theresa “Tree” Gelbman wakes up on a Monday, September 18th, on her birthday in the dorm room of classmate Carter Davis after spending the night before. The tree goes through the day being focused only on her, recalcitrant and mischievous with her colleagues, ignoring the invitation of her father’s restaurant and throwing a cake received from roommate Lori. That night, however, as she goes to a party, she is lured into a tunnel where she is killed by someone wearing a mask.
Tree wakes up the next morning in the same context as the previous one, and events repeat itself. Somewhat upset, she resigns the previous day and avoids that tunnel, going to another party where she combines with a classmate named Nick. But the masked murderer makes his appearance and, before he kills her again, he kills Nick.
Tree realizes that he is in a time loop and tries to avoid his own death barricading himself in his room. However, the killer finds a new way to take his life. During the next loop, she tells what happens to Carter, and he suggests taking advantage of the opportunity to find out who the killer is. In the following time loops, she tries to track down a few people she suspects might be the killer. After several failed attempts, Tree convinces Carter of the unpleasant situation he is in, and shows him that he has knowledge of the events that are going to happen throughout the day.