Laura is introduced to her friend Chloe's stepbrother, Harrison, who works at a record store called Lost & Found. Their connection is instant, and they decide to attend a concert by Japanese Breakfast together. During their time together, they meet Cooper, the record store owner, and share a special moment at the beach where Harrison gifts Laura a cassette aimed at creating a mixtape that symbolizes their bond. The pair celebrate the news of Laura’s acceptance into an art mentorship program, but Harrison expresses his intention to move across the country for medical school. This moment is cut short when an earthquake occurs.
Back at Laura's home, she shares her artwork with Harrison, but his parents break the news that he is set to embark on his medical journey. In a heartfelt moment, Harrison confides in Laura that he wishes to stay behind for her, but she encourages him not to sacrifice his dreams for their relationship. They attend Laura's art exhibition together, marking a milestone in their young lives. Unfortunately, tragedy strikes on Harrison's birthday when he is tragically killed by a car, leading Laura to destroy their mural in grief and leave her mixtape at Lost & Found.
Fast forward four years, and Laura finds herself at Chloe's wedding, where Cooper unexpectedly returns her mixtape. When she listens to it, Laura discovers that pressing play transports her back to moments spent with Harrison, starting with their first date at the concert. Each play offers her a brief visit to the past, during which she attempts to warn Harrison about the circumstances that lead to his untimely death. She forewarns him about a calamity that’s about to happen at the beach, successfully predicting the earthquake.
However, her actions begin to ripple through time. Laura learns that her interference has altered the present, as Chloe is now married to someone entirely different than her soulmate. Soon, she uncovers that Harrison died after falling off a cliff in her timeline. As she continues her journey through time, she goes back to the moment she revealed her artwork to Harrison, where she tries to get him to heed her warnings about avoiding the cliffs. Upon returning, she discovers that his demise was a result of electrocution.
Her next excursion takes her to a gathering at Lost & Found, but the timeline has already shifted — the store has burned down with Harrison inside, and the mixtape is now missing. In search of answers, she meets a despondent Cooper and retrieves the cassette from this altered timeline. Cooper expresses skepticism about whether she is meant to save Harrison, yet she is determined to try. Unfortunately, her efforts lead to another tragic outcome: this time, Harrison and his father die in a car accident.
Refusing to give up, Laura presses play again and arrives at her art exhibition, where she advises Harrison to break up with her in favor of pursuing his medical career. In her present time, her calls to Harrison go unanswered, leading her to seek solace in the mixtape once again. She finds herself at the moment they completed their mural, where Harrison reveals he would rather stay with her than leave and face the inevitable.
Eventually, Laura realizes she must accept the reality that being with Harrison may not be possible. While at Chloe's, she discovers an additional song on the B-side of the cassette, which mysteriously allows her to return to the moment they were to meet for the first time. Choosing to save his life, she resolves not to enter Lost & Found. In the present, a joyful Chloe invites Laura to join her for Christmas dinner, revealing that she is now with her true soulmate. Harrison arrives at the gathering and introduces himself to Laura, suggesting that their paths are fated to cross anew.
