On November 9, Nyles wakes up next to his girlfriend Misty on the wedding day of Tala and Abe. At the reception, he delivers what appears to be an impromptu speech, bringing relief to Tala's sister, the intoxicated and unprepared maid-of-honor, Sarah. As Nyles and Sarah begin to bond, they discover that Misty has been unfaithful. Just as they are on the verge of intimacy, Nyles is unexpectedly shot in the shoulder with an arrow by an older man named Roy.
Despite his injury, Nyles crawls toward a mysterious amber light emanating from a nearby cave, cautioning Sarah not to follow. Intrigued, Sarah ventures into the cave and is suddenly pulled into a vortex, finding herself back on November 9. After confronting Nyles, he reveals that she is now trapped in the same time loop as him. In her attempts to escape—through methods like driving home or even suicide—she finds no success. As they spend countless days together, often lounging by the pool of a neighboring home, Nyles shares that Roy is a family man from Irvine who also fell into the time loop thanks to Nyles himself.
A flashback recounts the night Nyles met Roy, where they indulged in partying and drugs sourced from Trevor. Roy, blissfully unaware of his fate, enters the cave after Nyles introduces him to the idea of eternal enjoyment. Unfortunately, Roy now holds Nyles responsible for his predicament and occasionally visits Palm Springs to seek revenge on him.
Over numerous cycles, Nyles and Sarah camp out in the desert, where their imaginations run wild, even leading to a romantic encounter. The following day reveals that Sarah has been waking up in Abe's room after spending the nights of November 8 with him. He encourages her to sneak away before anyone notices. Overwhelmed by guilt, Sarah struggles to discuss their night together with Nyles, deeming all their experiences as ultimately meaningless. Their situation escalates when they are stopped by a police officer who turns out to be Roy. Following a heated exchange, Nyles confesses that he and Sarah were intimate multiple times prior to her time loop induction, something he had previously hidden from her. Enraged, Sarah impulsively runs in front of an oncoming truck, leading to her death and a reset of the day.
In the subsequent reset, Nyles searches for Sarah, eager to apologize, but their paths do not cross. As he lounges in Abe's bed with the groomsmen, he catches a scent of Sarah's hair mist on his pillow, leading him to realize they were together again the night before the wedding. Confronting Abe results in a physical altercation between him and Nyles. After enduring more resets, Nyles finally realizes his feelings for Sarah. He decides to seek out Roy, who has abandoned his vengeance and now leads a “perfect but imperfect” life, reliving the same day while missing out on his children’s growth. As they talk, Nyles asks Roy to kill him one final time to initiate the reset.
Simultaneously, Sarah reflects on the time loops she experienced without Nyles and confronts Abe regarding their affair, both expressing regret. Determined to break free from the cycle, she dedicates her days to studying quantum physics and general relativity in a diner. After much research, she attempts her hypothesis by sending a goat into the cave adorned with explosives, discovering that the goat vanishes upon reset.
In the next cycle, Sarah rouses Nyles and shares her belief that they can escape, suggesting they detonate themselves in the cave before the day restarts. Despite Nyles admitting his love for her, he expresses a desire to prolong their time together in the loop, where he feels secure in predictability. After leaving him alone, Nyles breaks up with Misty out of frustration.
During the wedding ceremony, Sarah delivers a heartfelt speech for Tala before preparing for her final plan—she makes a phone call and heads to the cave with explosives. In a bar, Nyles, realizing he cannot bear the thought of losing Sarah, rushes to join her, declaring he would rather face death together than lead a lonely existence in the loop. Their feelings are reciprocated in a heartfelt moment before Sarah activates the detonator.
The scene shifts to Nyles and Sarah once again relaxing in the vacant pool, only to be interrupted by the house owners—it's now November 10, indicating their grand plan succeeded. In a mid-credits scene, Roy approaches Nyles at the wedding, inquiring about Sarah's method of escaping the loop. When Nyles fails to recognize him, Roy's smile reveals that their strategy worked.
