Prior to the events of the anime, the three factions – the Garan Army, the Kiba Army, and the Hachiryoukaku (Aira Army), battle for total dominance of Machine Island, which they occupy. The island is basically a prison, isolated from the rest of the world. To assure total quarantine, a “Gravity Curtain” was formed within the island. Only machines created by the worldwide government WSO are capable of entry to the island.