During the final years of the Edo period, as the Tokugawa shogunate's power begins to waver, a young writer named Momosuke travels the countryside with a singular ambition. Bored with writing riddles for children, he now dreams of compiling a book of one hundred ghost stories, a hyaku monogatari. His journey to collect these eerie tales leads him to a chance encounter with a mysterious group who call themselves the Ongyou.

This trio consists of Mataichi, a small, bandaged man posing as a monk who is in fact a master trickster and con artist; Ogin, a beautiful and voluptuous puppeteer with a tragic past; and Nagamimi, a large, shapeshifting bird-caller with centuries of experience in deception. Far from being simple storytellers or exorcists, the Ongyou are agents of a brutal, karmic justice. They seek out individuals whose hearts are so darkened by heinous crimes, particularly murder, that they emit a palpable aura of evil. Using their skills in illusion, disguise, and psychological manipulation, the Ongyou construct elaborate schemes to expose these sinners. They trap their targets in scenarios where guilt and fear force a confession, after which Mataichi delivers a final, spiritual judgment, sending the condemned soul to the next world with a ritual phrase: Ongyo shitate matsuru.

Momosuke, who is dubbed "Author" by the group, finds himself repeatedly crossing paths with the Ongyou. Fascinated and horrified, he becomes an unwilling witness to their methods. In each encounter, he is forced to confront the worst of humanity: child murderers, jealous lovers, cannibals, and corrupt officials. While he is initially a naive idealist who sees the world in clear moral terms, his experiences with the Ongyou steadily erode his innocence. He struggles with the group's cruel and remorseless efficiency, questioning whether their vigilante justice is any better than the crimes they punish. Despite his repeated pleas to join them, they always refuse, warning him to turn back before he is consumed by the darkness he so eagerly seeks to document.

The series is structured around a collection of self-contained episodes, each focusing on a different yokai legend such as the Azukiarai or the Willow Woman, which Mataichi and his accomplices weaponize to trap their targets. However, these seemingly independent cases build toward a larger, overarching conflict. The Ongyou ultimately find themselves in opposition to their own enigmatic master, Kyogoku Tei. A being of immense power who feeds on human atrocity, Kyogoku Tei is constructing a mystical superweapon, the Flame Lance, which is fueled by ritualistic murder and suffering. His goal is to shroud the world in an eternal age of darkness. This final narrative arc forces Momosuke to abandon his role as a passive observer. He must make an irrevocable choice about his own morality and decide whether to stand alongside the flawed and fearsome Ongyou in a final battle to prevent a cataclysm born from the very evil they have spent their existence punishing.