In a world where every person is born without a fixed gender, their bodies gradually transform to match the gender they desire once they reach the age of twelve. By fourteen, the transition is complete for most. But Arima Hinase, now eighteen and in their third year of high school, has never undergone any change. They carry a quiet unease as the only person around them who remains genderless, yet they have always found comfort in the unchanging days spent with their two childhood friends: the bubbly and outgoing Ritsu Kaga, who grew into a girl, and the thoughtful, gentle Shiori Takayama, who became a boy. That fragile equilibrium shatters when, on the same day, both Ritsu and Shiori confess their romantic feelings to Hinase. Hinase, who had never considered romance or their own identity beyond the present, is suddenly forced to confront questions they have long avoided. What does it mean to love someone when you have no gender? Who are they, and who do they want to become? As Hinase navigates these confessions, they begin to explore the possibilities of their own future, supported and challenged by classmates and adults who each struggle with similar uncertainties in their own ways. The story follows Hinase’s emotional journey of self-discovery, the shifting dynamics of the love triangle, and the quiet but profound moments of hesitation, growth, and choice that define their eighteenth spring.