Haruki Mishima, a high school freshman and swimming club member, carries a childhood memory that shaped his life—a kindergarten encounter where a girl read him a fairy tale and gifted him the book. Years later, he learns the girl was Misora Mashiki, the late twin sister of his classmate Miharu Mashiki. Reuniting with Miharu at a convenience store near their school, he grapples with burgeoning romantic feelings despite her fragile health and his own hesitancy to confront emotions.

Initially indecisive and prone to sidestepping emotional challenges, Haruki’s bond with Miharu sparks gradual growth. He often suppresses his desires to prioritize her needs, a stark contrast to his assertive friend Towa Honda’s romantic pursuits. Both navigate their relationships through shared moments at the convenience store, though Haruki’s path remains fraught with quiet internal struggles.

As Miharu’s health declines, Haruki’s resolve strengthens. He visits her daily during her hospitalization and, six months after their reunion, finally references their shared past by mentioning the childhood book. This marks his shift from passivity to purposeful action, balancing his earnest demeanor with newfound emotional openness. Their relationship deepens into mutual support, weathering hospital stays and impending surgery.

Haruki’s surname, combining "three" and "island," and his given name, meaning "spring" and "coming," mirror his journey—a transition from observer to active participant, echoing themes of renewal and forward momentum.