In a world long ravaged by the collapse of human, demon, and beast realms, the Overfiend awakens prematurely and braces for the arrival of his destined rival, the Lord of Chaos. Amano Jyaku and his sister Megumi travel to the kingdom of Azuma to investigate the growing signs of this threat. There they are drawn into the schemes of the dictator Caesar as a slave revolt stirs among the evolved half-breed demon beasts and an ominous power returns from the past. With the demon beast Buju at their side, the fate of the realms rests on their actions.