Setsuko and Seita are brother and sister living in wartime Japan. After their mother is killed in an air raid they find a temporary home with relatives. Having quarreled with their aunt they leave the city and make their home in an abandoned shelter. While their father's destiny who was a soldier is unknown the two must depend on each other to somehow keep a roof over their heads and food in their stomachs. When everything is in short supply, they gradually succumb to hunger and their only entertainment is the light of the fireflies.