Kippei and Keita have been a couple for over ten years, but their relationship has lost its spark. They struggle to communicate honestly, and their physical intimacy has nearly vanished. Kippei accidentally breaks a sentimental mug that he and Keita made together early in their relationship. The morning after this accident, Kippei inexplicably swaps places with his own eighteen-year-old self from a decade earlier.

This sudden time displacement forces the couple to confront their problems from two different angles. The twenty-eight-year-old Kippei finds himself in the past, face to face with the younger version of Keita. Meanwhile, the present-day Keita wakes up to discover the eighteen-year-old Kippei has arrived in the future to take his older boyfriend's place. Both men are given a rare opportunity to revisit the beginning of their love story and interact with the person they first fell for.

The story unfolds entirely through the perspectives of these two men at two different ages. Kippei struggles with feelings of inadequacy, convinced he must be perfect for Keita, whom he sees as more brilliant and competent. This insecurity has led him to rely on alcohol to ease his embarrassment about physical intimacy. Keita, abandoned by his parents as a child, lives in constant fear of being left behind. This fear manifests as a distant perfection that only reinforces Kippei's insecurities. Through encounters with their younger selves, each man discovers where their communication broke down and what they have been missing in their relationship.

The narrative arc follows the time-swapped scenarios as both pairs explore their connection from fresh perspectives. The younger Keita, inexperienced and enthusiastic, teaches the older Kippei something about the raw affection he has forgotten. The younger Kippei, innocent and awkward, reveals to the older Keita how his own actions inadvertently contributed to their problems. The broken mug serves as the symbolic catalyst for this journey, representing the fragile, handmade history the couple shares and the possibility of repairing something that seems permanently damaged. By experiencing their love from the outside, Kippei and Keita must decide whether they can piece their relationship back together.