|“|| Why do fireflies have to die so soon?
When a fleet of American B-29 Superfortress bombers are flying overhead, she and Seita are left to secure the house and their belongings, allowing their mother, who suffers from a heart condition, to reach a bomb shelter. Although they survive unscathed, their mother is caught in the air raid and is horribly burned. After she dies in a makeshift clinic in a school shortly afterwards, Setsuko leaves with Seita on their way to the home of a distant aunt, who later becomes increasingly resentful their food rations continue to shrink due to the war.
After Setsuko leaves with Seita again and moves in with him in an abandoned bomb shelter, they release fireflies into the shelter for light. However, the next day, Setsuko finds them all dead, and becomes horrified. She digs them a grave and buries them all, asking why they have to die, and why her mother had to die. What begins as a new lease on life grows grim as she and her brother run out of rice, and she is rushed to a doctor after becoming increasingly ill from malnutrition. She is found dying and hallucinating as Seita returns to the shelter with large quantities of food. She then dies as he hurries to cook, and is cremated the next day, after a montage of past moments in her life set to "Home Sweet Home" performed by Amelita Galli-Curci (1882 – 1963).
Setsuko is slender 4-year old girl. She has dark brown hair cut short above her ears, and shiny dark brown eyes. She wears an off-white polo shirt and blue monpe, and a hooded cape of the same color and pattern and off-white bloomers along with tabi socks and geta's (clogs). She also wears a red dress, and a white seifuku with black shoes and short socks in various flashback scenes.
- Seita Yokokawa (brother) (deceased)
- Kiyoshi Yokokawa (father deceased)
- Mrs. Yokokawa (mother deceased)
- Seita's Aunt (alive)
- Seita's Cousin
Setsuko Yokokawa was born in 1941 but later dies in September 1945 towards the end of the film.