On November 9, Nyles wakes up next to his girlfriend Misty on the wedding day of Tala and Abe. At the reception, he delivers a seemingly impromptu speech, to the relief of Tala’s sister, the drunk and unprepared maid-of-honor Sarah. Nyles and Sarah bond and, after discovering Misty cheating on Nyles, are about to have sex when Nyles is abruptly shot in the shoulder with an arrow by an older man named Roy. Wounded, he crawls toward a mysterious amber light in a nearby cave, warning Sarah not to follow. Sarah, curious, is sucked into a vortex in the cave.
Sarah wakes up, again, on November 9. After she confronts Nyles, he explains that she has become stuck in the same time loop as him. Sarah unsuccessfully attempts many escape methods, such as driving back to her hometown or suicide. Nyles and Sarah spend many days together, often relaxing in the pool of a home where the occupants are away on vacation. Nyles reveals that Roy is a family man who lives in Irvine, whom Nyles also trapped in the time loop. A flashback shows how Nyles and Roy meet, party, and do drugs, which they get from Trevor; Roy wishes that the night would never end, and an inebriated Nyles introduces him the cave. Roy, unaware of the time loop, goes in. Nyles explains to Sarah that Roy blames him for getting stuck in the time loop and occasionally comes to Palm Springs to hunt Nyles for revenge.
After many more loops, Nyles and Sarah camp out in the desert, where they hallucinate about dinosaurs, and eventually have sex. The next day, it is revealed that Sarah has been waking up in Abe’s room having spent the nights of November 8 with him, and he advises her to leave before she’s seen. A guilt-ridden Sarah refuses to talk to Nyles about their previous night, expressing that everything they do is meaningless. They are pulled over by a police officer, who turns out to be Roy. Sarah runs over Roy in his police car, and after an argument, Nyles admits that he had sex with Sarah numerous times before she entered the time loop, something that he had lied about before. A disgusted Sarah runs in front of an oncoming truck, killing herself and restarting the day.
After the next reset, Nyles goes looking for Sarah to apologize, but he cannot find her. Lying in Abe’s bed while the groomsmen take drugs, Nyles smells Sarah’s hair mist, Orchid Explosion by Fournier, on his pillows and realizes that Sarah and Abe had sex the night before the wedding. He confronts Abe during the wedding, and a fight breaks out between Nyles and Abe. Multiple resets later, Nyles realizes that he loves Sarah. He travels to Irvine to talk to Roy, who has given up on killing Nyles. Roy explains that his life is perfect but also imperfect, as he lives in the perfect day forever, but will never get to see the future or his children grow up. Roy says that they should never speak again, and Nyles asks Roy to murder him one last time to jump-start the “reset”.
The time loops where Sarah was gone are then shown, Sarah confronts Abe over their affair, and both express their regret and remorse. Sarah then resolves to escape the time loop, spending many days in a diner studying quantum physics and talking with professors, finally coming to a solution. She tests her theory by sending a goat into the cave with explosives. The next reset, the goat disappeared. Sarah wakes Nyles up and tells him she believes she knows how to escape the time loop by blowing themselves up in the cave before they restart the day. Nyles admits he loves Sarah but wants to stay in the time loop forever, where he knows how everything will work precisely. She leaves him in the room alone, and a frustrated Nyles breaks up with Misty.
Sarah attends the wedding, gives a heartfelt speech for Tala, makes a phone call, and goes to the cave with explosives. Drinking alone in a bar, Nyles decides to leave with her and rushes to the cave, professing he would rather die with her than remain in the loop alone. Sarah reciprocates his feelings, and they kiss while she presses the detonator. In the final scene, Nyles and Sarah are once again lounging in a vacant pool when the house owners show up, revealing it is November 10, and the plan worked.
In a mid-credits scene, Roy approaches Nyles at the wedding, asking about Sarah’s plan to escape the loop. When Nyles does not recognize Roy, Roy realizes the plan has worked and smiles.