Following his mother's death, Yuuta Morisaki moves to the city with his older sister Ruri. Enrolling at a local high school, a hallway encounter with student Hina Kusunose triggers fragmented memories of a childhood friendship. This meeting sparks the slow return of his recollection of a promise made to a girl in his early years, though he doesn't initially recognize Hina as that person.

Ruri lives with Yuuta, and their relationship experiences moments of friction. At school, Yuuta joins the photography club, befriending a classmate fascinated by candid shots of female athletes. Through this connection, Yuuta encounters several female classmates who develop romantic feelings for him, largely unnoticed by Yuuta himself.

His character arc centers on the painstakingly slow recovery of childhood memories involving Hina. While he clearly recalls other figures from his past, the memories of Hina and their shared promise return only in pieces through their interactions. This memory gap creates tension, as Hina remembers him while he grapples with her significance.

Yuuta's journey culminates at the culturally significant Ocean Festival, known for romantic traditions. Amidst the festival activities and fireworks, his fragmented memories finally coalesce. He fully recalls the specific childhood promise made to Hina. This realization coincides with his acknowledgment of romantic feelings for her, leading to their mutual confession at the festival grounds.