The Devil King Lucifero suddenly announces his intention to retire, necessitating the selection of a successor. The Devil King Cadets assemble one by one and travel to the human world to compete in the succession contest. However, Osamu, the Devil King's own descendant, has absolutely no interest in the competition. Rather than the fate of the world, Osamu is more concerned with his part-time job at a convenience store. Nevertheless, Osamu is forced to begin collecting familiars known as "Child," which are born from human souls. Osamu remains focused solely on his part-time job, placing him a few steps behind the other Cadets.