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 soldier father's destiny 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...