On April 18, 2003, the vibrant high school senior Summer Field eagerly anticipates the upcoming school dance, Spring Fling, despite the tragic shadow looming over her. Only days before, her dear friends Emmy, Brian, and Val were brutally murdered by the infamous serial killer known as The Sweetly Slasher. In a rushed attempt to maintain some semblance of normalcy, Summer brushes off her childhood friend Quinn, who tries to present her with an envelope, and she rejects a dance invitation from her recent ex, Ethan.
As the evening progresses, chaos ensues when the police arrive to shut down the party. In a moment of panic, Summer retreats to a back bathroom, only to be confronted by a menacing figure brandishing a knife. Fleeing for her life, she bolts towards the barn but unfortunately succumbs to the attack.
Fast forward two decades later, and we meet Lucy Field, Summer's high school-aged sister who feels like a ghost in her own town, overshadowed by the tragic legacy of her sister's murder. The community is haunted by memories of The Sweetly Slasher, with many of its residents still grappling with the gruesome events that transpired. Lucy, left to navigate this bleak environment, finds herself accompanying her parents to a shrine honoring the victims near the barn, a stark reminder of their collective loss.
While exploring, Lucy stumbles upon a time machine hidden in a hay barn, a discovery that sends her spiraling back to the fateful year of 2003, specifically just two days before Summer's tragic demise. Confused by the nostalgia of a town that seems much more lively and unblemished, she heads to her high school for guidance, seeking her mentor, Mr. Fleming, for advice on time travel. However, when Quinn overhears her inquiry, he adamantly warns her against such reckless endeavors.
In a turn of events, a senior prank leads to Quinn being bullied, prompting Lucy to spring into action. At first, Quinn is annoyed by her intervention, but Lucy confides in him about her accidental arrival via the time machine. Recognizing his aptitude in physics, she enlists his help to refuel her time device.
While staring at the vibrant life of her sister and her family, Lucy realizes how drastically different things were back then. Summer's home is filled with warmth and laughter: her mother teaches her the joys of cooking while Summer blossoms into a young woman. Tragedy soon strikes again, however, when the Slasher claims the lives of Val and Brian; Lucy's attempt to alter fate backfires, leading to the unintended death of a security guard.
As the story unfolds, Summer becomes aware of the danger lurking around her, joining forces with Lucy and Quinn to protect Emmy from the pernicious killer. During this chaotic journey, Lucy learns about the depth of Quinn's feelings for Summer, learning that the letter he tried to give her all those years ago was a heartfelt confession of love. In a climactic moment, Lucy and Quinn, while in pursuit of fuel for the time machine, bear witness to Summer's blossoming relationship with Emmy, who bravely declares her love for Emmy at the dance.
The Slasher's true identity is finally revealed—an alternate version of Quinn, consumed by jealousy and mental anguish due to the bullying he faced in his own timeline. In a twisted chain of events, this alternate Quinn traveled back through time, seeking revenge that would irrevocably alter the lives of Summer and her friends. In a desperate bid for survival, Lucy manages to transport both her and the future Quinn into 2024, where she confronts him, leading to his demise through quick thinking and a clever trap.
Realizing that her existence has vanished in this brighter new timeline, Lucy makes the courageous choice to return to 2003, yearning to embrace her life alongside Summer and Quinn once again. Armed with newfound optimism, she seeks an internship with NASA, reminiscent of the opportunity she had hoped for in her original timeline, revealing her revived hope for the future in her personal statement. Through her journey, Lucy not only strives to protect her sister but also learns to cherish family, friendship, and the possibilities that life can offer.
