The story is set in a distant future where Japan is divided into feudal territories and conquered by other countries. There is a floating city named Musashi that is composed of eight ships in the skies. A war that will determine the fate of the world will be fought by students on the Musashi. In the far future, humans abandon the devastated Earth and move to the upper world "Tenjou." But Tenjou and the law of causation collapse due to a war and humans are forced to return to the Earth, which has turned into an inhabitable planet except for Shinshuu area in the Far East. Shinshuu is too small to accommodate all the humans, so they duplicate the area and create "Juusou Sekai" in a parallel world. In order to rebuild Tenjou and the law of causation, the returned humans start reproduction of