Stillman first crossed paths with Debbie, played by Sophie Turner, six months ago at a bar where he frequently visits with his friend Evan, portrayed by Skyler Gisondo. Debbie, an aspiring musician working as a waitress, struggles with remembering orders and lacks a clear direction for her future. Rather than having a structured plan, she embraces life in the moment, having recently taken a trip to Hawaii, driven by an urge to simply be there.
Stillman, a physics prodigy played by Asa Butterfield, appears destined for greatness but finds his trajectory thwarted when Debbie abruptly ends their relationship. Struggling to make sense of the split, he creates an interactive map detailing their dating history, with smiley faces marking joyful moments and red faces representing the unhappy ones. As he ponders his missteps, Stillman resolves to invent a time machine, determined to win back Debbie.
His ingenious device allows him to transport his present consciousness back into his own past self, with the help of an app controlled via his smartphone. Alongside Evan, Stillman embarks on a journey to revisit the pivotal instances in their relationship that led to Debbie's breakup. Despite his various approaches—embracing acceptance, attempting denial, or using reverse psychology—nothing seems to sway Debbie, and she remains committed to her decision to part ways.
Recalling a bittersweet moment when Debbie penned him a song after he confessed his love, he attempts to decipher her feelings through the lyrics but finds himself lost in confusion. They revisit the day they first met, allowing Stillman to replay that moment endlessly, reflecting on prior interactions that went wrong, such as when Debbie mockingly critiqued his favorite movie, 'Farallon Five.'
Even as Stillman learns that silence might be his best tool, he encounters various moments—one involving a roof incident where he impressively uses a pocketknife to liberate them—that reignite sparks between them. Eventually, during their mission, the app malfunctions, and Stillman is left with no choice but to navigate forward in time instead of reliving moments.
Opening up during a heartfelt conversation, Stillman expresses concern over Debbie's friend Ryan, played by William Peltz, trying to pursue Debbie. But Debbie firmly defends Ryan, insisting he is gay. Following Debbie's performance at a bar, where she receives an offer to play regularly, Stillman chooses to remain silent instead of offering unsolicited advice, allowing her to make her own choices.
As Evan starts to change his dynamics, even experiencing newfound success in a Frisbee tournament against his rival Andy, he and Stillman witness the fabric of their lives transform with every adjustment they make. They reach a new phase where tense situations evolve into positive outcomes, and as they continue pressing forward, Stillman discovers that Debbie has indeed chosen to stay with him.
Returning to the present day, Stillman is overjoyed to find Debbie still by his side. However, mundane disagreements soon lead to chaos, including a birthday spat about tardiness. Faced with a deteriorating relationship, he realizes his dependence on the time machine is creating an illusion of perfection that ultimately disturbs Debbie.
After a suggestion from Debbie about having a baby, panic ensues with Evan, who struggles with maintaining the machine's secrecy. Their friendship frays under tension, but as they grapple with the past, they find themselves reliving the moments leading to their very first meeting. As emotions intensify, Stillman and Debbie's future selves engage in conflict before finding reconciliation once more, illustrating the complexity of love and the unavoidable messiness that comes with it.
