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 comes to 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 Monday 18 the day 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 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.
Tree wakes up in Carter's room on Monday the 18th, and relives her original time loop, but she soon realizes things are different after finding out that Carter is now dating a nicer Danielle. Ryan theorizes that the reactor caused Tree to drift into another dimension. When Tree learns her mother is still alive in this new reality, she decides to stay.
That night, Tree goes to the hospital to intercept serial killer John Tombs before he escapes but is confronted by a police officer. Babyface kills the officer, and Tree runs into Lori at the elevator, realizing she isn't the killer in this reality. Lori tells Tree that Babyface cannot be Tombs because she just took him in for surgery. Babyface stabs Lori, then chases Tree to the roof, where she accidentally falls to her death. She wakes at the beginning of her loop and demands that Ryan and his team help her escape it, requiring they test dozens of algorithms. At Carter's suggestion, Tree serves as the group's recorder, killing herself at the end of each day so they can start again. Eventually, her injuries catch up with her, and she faints. Waking up in the hospital, Tree steals a gun to go after Tombs, only to find Lori already dead. Tombs, disguised as Babyface, attacks, and Tree shoots him dead. However, a second Babyface appears, forcing Tree to kill herself and Babyface.
The group finally discovers the correct algorithm, but a technical issue forces a delay. Faced with a choice of which reality she wants to be in when both time loops close, Tree decides to remain in the current dimension. Carter urges Tree to consider the consequences of living a life that is not truly hers, and states that her experience with grief helped shape the person she is now. Tree hides from Babyface in a hotel. That evening, the news reports that Carter was murdered trying to save Lori at the hospital. Tree kills herself by exploding a power station, deactivating the reactor so she can save Carter and Lori. The loop restarts and Tree decides to return to her own reality. She advises Lori to end her affair with her professor Dr. Butler, discovers that Danielle is cheating on Carter and has a final conversation with her mother.
Bronson confiscates the reactor before the group can activate it. Believing she is too weak to survive another loop, Tree insists they retrieve the device. The group enlists Danielle to distract Bronson while they recover the reactor. As Ryan readies the device, Tree goes to the hospital to rescue Lori from Tombs, but is trapped by the second Babyface—revealed to be Dr. Butler trying to bury the evidence of his affair with Lori. Dr. Butler's wife Stephanie appears and shoots Lori, revealing she is in league with her husband, but he betrays and shoots her as well. Tree outsmarts Butler and kills him. Lori survives, and Tree and Carter kiss as the reactor activates, sending Tree back to her original dimension on Tuesday the 19th.
Later, Tree, Carter, Ryan, Samar, and Dre are escorted by agents to a darpa laboratory, where the reactor has been moved for further experimentation. When the agents say they need a test subject in order to see how the machine works, Tree says she knows the perfect test subject. In her bedroom, Danielle wakes up, screaming in horror.
