College student Tree Gelbman finds herself once again trapped in a terrifying time loop where she repeatedly lives the same day, the day of her own murder, which she thought she had escaped from for good. However, this time the circumstances have changed: the loop is not exactly the same as before. In her new ordeal, Tree discovers that her friends, including her boyfriend Carter, are now involved in the loop too, particularly Carter's roommate, Ryan, who is being repeatedly killed by a mysterious masked figure. As Tree struggles to understand why she is once again stuck reliving the same day, she learns that Ryan and his fellow science students have created an experimental quantum reactor which appears to be the source of her time loop predicament.

As Tree navigates her new reality, she faces a difficult choice: accept the new timeline with all of its alterations and hardships, or risk everything to try and return to her former life, despite the dangers that might await her. Each reset brings new insights into the lives of those around her and uncovers alternate possibilities. With knowledge acquired from each loop, Tree and her friends attempt to solve the puzzle of why the loop has restarted and how to stop it once and for all, as well as the identity of the new killer hunting them. Confronting emotional challenges and impossible choices, Tree must use her wits and strength to overcome the horrors of the past and embrace a future she's yet to understand.