The plot centres around William, the child of an aristocratic family with exceptional intelligence. His uncle's valuables were lost one day after his firm failed. Fearing that his family's reputation has been tainted, William goes home and hunts for anything that may be transformed into cash with the help of his family's butler. A search of the properties turns to an ancestor's underground chamber. There is a mystical seal in the room, and William unwittingly summons the devil. The summoned devil introduces himself as Dantalion and discloses that William is the designator who has the power to determine the acting king of the demon world.