Happy Death Day 2U (2019)

One day after the events of Happy Death Day, college student Ryan Phan wakes up in his car on September 19. Returning to his dorm room, he walks in on his roommate Carter and Carter's girlfriend, Tree.

Ryan resumes work on an experimental quantum reactor with fellow students Samar and Dre to slow down time. After Dean Bronson shuts down the project for causing several power outages and not producing practical applications so far, Ryan is murdered by someone dressed as school mascot Babyface and wakes up again on the 19th.

Tree explains her experience reliving September 18th, and she and Carter agree to help Ryan. They learn the reactor inadvertently created the loop. The masked killer tracks Ryan down, but Tree unmasks him to reveal another Ryan. The second one insists the original must die for the loop to close. Terrified, Ryan activates the reactor, releasing an energy pulse that knocks everyone unconscious.

Tree wakes up in Carter's room on September 18 and relives her original time loop, but she soon realizes things are different after discovering that Carter is now dating Danielle. Ryan believes the reactor caused Tree to drift into another dimension. When she learns her mother is still alive in this new reality, she wants to stay.

That night, Tree goes to the hospital to intercept serial killer John Tombs before he escapes, but she is confronted by a police officer. The killer kills the officer, and Tree runs into Lori at the elevator, realizing Lori is not the killer in this reality. Lori insists the killer cannot be Tombs as she just took him in for surgery. The killer stabs Lori, then chases Tree to the roof, where she accidentally falls to her death. Waking at the beginning of her loop, Tree 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 restart. Eventually, her injuries catch up with her, and she faints. Waking in the hospital, Tree steals a gun to go after Tombs, only to find Lori already dead. He attacks, disguised as Babyface, and Tree shoots him dead. However, a second Babyface killer appears, forcing Tree to kill herself and the killer.

The group finally discovers the correct algorithm, but a technical issue forces a delay. Having to choose which reality she wants to be in when both time loops close, Tree decides to remain in this 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 who she is now.

Tree hides from the killer in a hotel, but that evening, the news reports that Carter died 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 Gregory Butler, discovers that Danielle is cheating on Carter, and has a final conversation with her mom.

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. They enlist 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 killer—revealed to be Gregory trying to bury the evidence of his affair with Lori. Gregory's wife Stephanie appears and shoots Lori, revealing she is in league with her husband, but he betrays and kills her also. In the ensuing confrontation, Tree outsmarts Gregory and kills him. Lori survives, and Tree and Carter kiss as the reactor activates, sending Tree back to her original dimension on September 19.

Later, Tree, Carter, Ryan, Samar, and Dre are escorted by agents to a DARPA laboratory, where the reactor has been moved for further experimentation. As they need a test subject, Tree volunteers Danielle, who wakes up in her room, screaming in horror.