In war-torn Kobe, Japan, the home of teenager Seita and his younger sister Setsuko is destroyed in a firebombing. Their father is serving in the navy and their mother dies from severe burns. Seita and Setsuko move in with a distant aunt but she turns out to be mean and cruel. Seita finds a cave for shelter and he and his sister are delighted as fireflies provide them with light. Setsuko becomes ill and dies. Seita cremates her body and puts the ashes in an empty candy tin. Shortly after World War II ends, Seita dies of starvation. A janitor finds Seita’s body. He takes the candy tin and throws it in a field scattering Setsuko’s ashes. They are both dead, brother and sister, but their spirit survives as they sit on a bench overlooking present-day Kobe surrounded by fireflies.