Koko, living alone in Tokyo, is suddenly visited by her childhood friend Shota, who used to live in her hometown. Though she always treated him like a kid since he was younger, she gradually finds herself drawn to him as he protects her from a persistent man and starts acting more mature. Her heart begins to race just like it did in the old days, and though she's surprised by his transformation, she can't help but be moved. Finally, Shota confesses his feelings, saying he wants to fulfill their childhood promise: "Let's start dating when we grow up."