At Sannomiya Station on 21 September 1945, shortly after the end of World War II. A boy, Seita, is dying of starvation. Later that night, having removed Seita's body, a janitor digs through his possessions and finds a candy tin which he throws away into a nearby field. The spirit of Seita's younger sister, Setsuko, springs from the tin and is joined by Seita's spirit as well as a cloud of fireflies. Seita's spirit then begins to narrate their story accompanied by an extended flashback of the final months of World War II.