The show is set in a futuristic universe, controlled by a few races or "tribes" that possess the capabilities for interstellar travel. The universe had once been ruled by a "Golden Tribe" (黄金の種族 "Ougon no Shuzoku"), who had since left the current galaxy long ago; they passed on their knowledge to the humanoid , the insectoid , and the gigantic . The latter was later punished by the Golden Tribe for causing havoc in the universe and made to serve the other tribes as - 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 . Viewed as a threat to the order of things, the Silver Tribe decided to annihilate the Iron Tribe with the aid of the Bronze Tribe. However, humanity survived their assault on Earth and scattered their numbers across the stars in order to preserve their race.