After Lucifero, the underworld's ruler, announces his retirement, the search for his successor commences. Many Devil King Cadets flock to the human world, hoping to win the coveted position. However, Osamu, who is the only descendant of Lucifero, doesn't share their enthusiasm. He's more interested in his part-time job at a convenience store than the fate of the world. Despite this, Osamu must collect "Child," powerful creatures born from human souls, to participate in the contest. Unfortunately, his lack of interest makes him lag behind the other Cadets.