The show is set in a futuristic universe, controlled by a few races or "tribes" that possesses the capabilities for interstellar travel. The universe had once been ruled by a "Golden Tribe", who had since left the current universe long ago; they passed on their knowledge to the humanoid "Silver Tribe" (銀の種族, Gin no Shuzoku?), the insectoid "Bronze Tribe" (青銅の種族, Seidou no Shuzoku?), and the gigantic "Heroic Tribe" (英雄の種族, Eiyuu no Shuzoku?). The latter was later punished by the Golden Tribe for causing havoc in the universe and made to serve the other tribes as 'Nodos' (ノドス, Nodosu?) - extremely powerful beings that play a key role in the story. Before the Golden Tribe departed, Humanity answered the call of the Golden Tribe and was dubbed the "Iron Tribe" (鉄の種族, Tetsu no Shuzoku?).