As college student Ryan Phan begins his fateful day on September 19, he suddenly finds himself back in his dorm room for an unexpected reunion with his roommate Carter Davis and his girlfriend Tree Gelbman. Following the events of Happy Death Day, Ryan’s goal is to utilize his inventive mind to collaborate with fellow students Samar Ghosh and Dre Morgan on an ambitious quantum reactor project designed to manipulate the flow of time. Their academic adventure, however, faces a major setback when Dean Roger Bronson intervenes, shutting it down due to troubling power outages and a lack of tangible benefits.
Despite these obstacles, Ryan meets a tragic end at the hands of a murderer disguised as Babyface, only to wake up once more on September 19. With Tree revealing that she also relived September 18, she sets her resolve to delve into the mystery of Ryan’s looping fate.
As the duo digs deeper into the baffling time loop, they uncover the shocking truth: their experimental reactor triggered this temporal glitch. The masked assailant continues to stalk Ryan, but when Tree peels back the mask, a startling revelation emerges—another version of Ryan is the true killer. This alternate Ryan warns him of the need to perish for the loop to end, sending waves of dread through him. In a desperate attempt to shatter the cycle, Ryan activates the reactor, unleashing a powerful energy pulse that sends everyone crashing.
When Tree awakens in Carter's room on September 18, she realizes she is reliving her original time loop, albeit with slight variations. The surprising revelation that Carter is now dating Danielle instead of her prompts Tree to recognize that she has slipped into an alternate reality. As Ryan theorizes about the reactor's capability to alter the fabric of reality, Tree focuses on maintaining her new life where her mother is still alive. With the stakes higher than ever, she wrestles with the painful choice of either sacrificing her chance for a different existence or closing the loop.
As night approaches, Tree races against time to thwart John Tombs’ escape from the hospital. When she inadvertently encounters a police officer who meets a brutal fate at the hands of the killer, she faces Lori Spengler in an elevator. In a moment of clarity, Lori reassures Tree that Tombs has already been prepped for surgery, turning on him as the true threat. Just as they verbalize their confidence, the killer strikes, and Tree finds herself sprinting to the rooftop, resulting in her tragic demise.
Reborn at the start of her temporal loop, Tree compellingly demands that Ryan and his team rally together to liberate her from this looping nightmare. Her persistence fuels their experimentation, leading to Carter's suggestion that Tree document their efforts which proves a crucial breakthrough. Each day, Tree’s self-sacrifice accumulates wounds that ultimately exhaust her. When she abruptly regains consciousness in the hospital, she seizes the moment to confront Tombs, going undercover only to find Lori already dead. Facing Tombs, now impersonating Babyface, she displays unwavering resolve, leading to a heartbreaking clash where she must kill both him and her.
As the group approaches the solution to escape the relentless loop, a technical malfunction delays their escape plan. Faced with choosing which reality to claim after the cycle, Tree makes the harrowing choice to stay in her current dimension. Carter urges her to reflect critically on the repercussions of choosing a life that isn’t rightfully hers, linking it to her profound journey through grief.
Amidst the chaos, Tree searches for safety in a hotel room, only to find a news report celebrating Carter's noble sacrifice at the hospital which saved Lori. Determined to rewrite the narrative, Tree sabotages the power station’s reactor, securing safety for both Carter and Lori. As the loop resets, Tree chooses to return to her original reality, sharing wisdom with Lori to end her affair with Gregory Butler and confront Danielle's betrayal towards Carter. A final exchange with her mother empowers Tree to start anew, brimming with the knowledge of what lies ahead and the strength to forge a new destiny.
As Dean Bronson takes control of the reactor, Tree's fierce ambition to avoid another loop leads her to insist on the retrieval of the device for guarantee of her existence. Voicing her fragility against another iteration, she pleads with her friends to act swiftly to acquire the crucial element. Enter Danielle, who distracts the foe, allowing Ryan to prepare the reactor while Tree sets out to save Lori from Tombs. Nonetheless, Ryan's plan faces an unexpected hurdle when Dr. Butler surfaces as another Babyface killer, driven by a desperate need to hide his illicit relationship with Lori.
In the ensuing mayhem, Stephanie Butler arrives with chilling precision, escalating the chaos precipitated by her husband’s betrayal and murder of his accomplice. In the heat of the moment, Tree outsmarts her adversary, executing revenge that claims Dr. Butler. Lori emerges bruised but alive, and Tree shares a heartfelt moment with Carter amidst the distress. As the reactor comes online, she is thrust back to September 19 in her original dimension.
The aftermath of this tumultuous journey sees Tree, Carter, Ryan, Samar, and Dre taken by government agents to a DARPA lab where the reactor is set for further tests. When officials indicate a need for a test subject to assess the device's performance, Tree seizes the moment, her gaze heavy with understanding as her fate hangs precariously in the balance. As Danielle awakens, her fragmented mind echoes the trauma she has endured, her screams piercing the abyss as the reactor looms ominously in the foreground.
