At the end of the 20th Century, the world is facing destruction from massive earthquakes, bad weather and other abnormal phenomena. In Tokyo, Setsuna Kai, a young boy who lost his mother when he was very small, practices Kendo every day because his goal in life is to become strong. As always, his rival Mirai Kaname defeats him and Setsuna feels sad. On his way home, a messenger from the Devil's World appears before him. This messenger proclaims that the various abnormal changes in the world are the work of the Devil. The world will come to an end unless the Devil is conquered. The only ones who can save the world are the "Devil Children." Thus begins Setsuna's adventure in the Devil's World.