On April 18, 2003, high school senior Summer Field attended the Spring Fling celebration, haunted by the recent murders of her friends by the Sweetly Slasher. Her friend Quinn tried to give her a letter, but she rejected it, just as she had rejected her ex-boyfriend Ethan's dance invitation. As the party ended, a masked figure attacked Summer in a bathroom, chasing her into a nearby barn.

Twenty years later, Summer's younger sister Lucy, still grappling with the trauma of Summer's death, visited the victims' memorial. She discovered a time machine in a barn and traveled back to 2003, two days before Summer's murder. 

Disoriented by the vibrant atmosphere of Sweetly, Lucy sought guidance from her science teacher, Mr. Fleming. Quinn, overhearing her discussion about time travel, warned her of its risks. When Quinn became the target of a senior prank, Lucy intervened, saving him and revealing her time-traveling abilities.

Lucy confided in Quinn, who agreed to help her refuel the time machine. She went to her own house, pretending to help Summer with homework, observing her family in a happier state. 

The Slasher murdered Val and Brian, but Lucy's attempt to intervene resulted in an accidental death. Summer eventually discovered the truth and worked with Lucy and Quinn to save Emmy.

During this, Lucy learned about Quinn's feelings for Summer through the letter he had tried to give her. As Lucy and Quinn acquired fuel for the time machine, Summer publicly expressed her love for Emmy at the dance. The Slasher attacked Summer, but Lucy and Quinn rescued her.

The Slasher was revealed to be a future Quinn from an alternate timeline, driven insane by past bullying and rejection. He traveled back in time seeking revenge, inadvertently creating Lucy due to the void left by Summer's death. Lucy returned to 2024 with future Quinn, killing him.

Finding that she didn't exist in this altered timeline, Lucy returned to 2003 to live with Summer and Quinn, pursuing an internship with NASA, embracing a more optimistic outlook on life.