Seiji Tohno possesses a troubling curse: he sometimes sees the people around him as terrifying monsters. This ability has driven him to the margins of society, leaving him a desolate NEET with no home, drifting between internet cafes just to survive. Everything changes one twilight evening when he stumbles upon a massive, incongruous European-style mansion. Inside, he meets Shiroshi Saijou, a bewitchingly beautiful boy dressed in a kimino adorned with a white peony pattern. Shiroshi seems to know more about Seiji than Seiji knows about himself, and he offers the desperate young man a deal: room and board in exchange for becoming his part-time assistant.

Seiji soon discovers that Shiroshis so-called proxy service is nothing less than an outsourced death-sending operation for hell. Shiroshi, who claims to be the son of a demon king, is tasked with identifying and consigning sinners possessed by yokai to eternal damnation. Seijis monstrous vision proves to be the perfect tool for this grim work, as it allows him to see the hidden corruption and spiritual possession lurking within ordinary people who have escaped justice. What Seiji sees as monsters are actually visual manifestations of peoples sins.

The pair function as a strange detective agency for the supernatural. Shiroshi acts as the cool, deductive, and enigmatic mastermind while Seiji serves as a frightened, often bumbling, but indispensable asset. Their cases are episodic and dark, forcing them to confront a host of horrors. One early case involves a pregnant woman receiving anonymous, terrifying messages urging her to hang herself, forcing Seiji and Shiroshi to unravel a mystery involving manipulation and supernatural terror. Another investigation takes them to a mountain ryokan burning with autumn colors, where they encounter a writhing serpent in the darkness and the corpse of the landlady, all pointing to a so-called serpents curse with tragic human motives.

As they delve deeper into their work, the mysteries become more personal and the stakes grow higher. Shiroshi receives a letter hinting at an impending murder and dismemberment, leading the pair to a remote island dominated by a towering Baroque mansion. There, they encounter a beautiful living doll and a young boy who claims to be Shiroshis brother, suggesting that the demon kings family is far more complicated and dangerous than Seiji ever imagined. Throughout these arcs, Shiroshi offers the sinners a chance to repent and acknowledge their crimes, but if they refuse, the price is an eternity in hell. The narrative is a rich blend of horror, folklore, and psychological deduction, exploring themes of guilt, justice, and the monstrous potential that hides within the human heart.