The last days of the Second World War. U.S. aircraft bombed the defenseless Japanese cities. A 14-year-old Saito and his little sister Setsuko get into the maelstrom of human everyday nightmare. Having suffered the bitterest loss – the loss of loved ones – they were all alone. The young boy instantly becomes an adult facing the cruelty of the world. He realizes that his little sister’s life depends on him. Finding a shelter in an abandoned asylum, Saito and Setsuko are trying to survive by relying on their own strength. And even a deadly fire in the sky is only the lights of fireflies in their eyes, the brutal war does not spare even the innocent children's souls, bringing them to their merciless sentence.