In the kingdom of Roseblade, the second Prince Abel possesses unusual physical strength and is beloved by commoners, but he is coldly treated by his father the king and his elder brother, the first Prince Strain, partly because of a peculiar birthmark on his neck. The queen and his mother, Retina, is the only one who showers him with genuine affection. One day, a mysterious knight in armor appears and drags Retina through a strange doorway into another realm. Desperate to save her, Abel encounters a demon named Leon, who offers him a contract that would grant him immense power. The ritual is interrupted and only half-finished, but Abel manages to cross the door and finds himself in the demon world.

The setting is a fantastical realm of violent hierarchies, where demon aristocrats and royalty hold overwhelming power. Leon is a demon of noble birth but ranks far below the ruling demon king and his family in both strength and status, making him a precarious ally. Abel quickly learns that his human might, which made him formidable in his own world, is almost useless here. To survive and search for his mother, he must rely on Leon's cunning, his own growing resolve, and the imperfect bond of their contract.

The narrative arc follows Abel and Leon traveling through the demon world, encountering a variety of demon inhabitants and uncovering the politics of its ruling houses. A key incident occurs when Abel gets into a confrontation with a demon royal at a shop; Leon intervenes to protect him and is forced into a battle against a far more powerful opponent. Through these clashes, Abel's understanding of the demon world deepens, and the relationship between the kind-hearted prince and the sly, powerful demon evolves beyond mere obligation. The story builds toward a confrontation with the demon king and the truth behind Retina's abduction, concluding with a resolution that tests Abel's humanity and Leon's loyalty in the harsh laws of the demon realm.