In Taisho-era Tokyo, the capital is terrorized by the phantom thief known as the Fiend with Twenty Faces. The boy detective Kobayashi, a sharp-witted youth who is actually a girl disguised in male attire, is determined to capture the mysterious criminal. His pursuit leads him to a peculiar idle young man named Akechi, a wealthy loafer who spends his days in a café, constantly complaining of boredom. Kobayashi becomes convinced that Akechi is the Fiend. Before he can prove it, the two stumble upon a brutal murder at a used bookstore in D. Hill. Kobayashi is wrongfully arrested as the prime suspect, and Akechi—reluctantly—solves the case, clearing Kobayashi’s name and revealing his own extraordinary deductive abilities.

It soon becomes clear that Akechi and the Fiend with Twenty Faces are the same person. Driven by a morbid fascination with the darkness lurking in people’s hearts, the Fiend alternates between committing audacious thefts and solving the very crimes he sometimes instigates. Kobayashi, having deduced his double identity, decides to become his assistant, hoping to channel his talents toward justice while keeping his criminal side in check. She disguises herself as a boy both to pursue detective work and to defy her conservative father.

Together, they confront a series of bizarre cases: a seemingly perfect murder solved through psychological manipulation; a wave of suicides in a girls’ dormitory caused by a hidden stalker in the attic; a kidnapping orchestrated by the “Black Hand Gang”; and a family’s bloody revenge under the alias “Issun-boshi.” Each case peels back layers of human depravity, and the Fiend savors every revelation. As the stories progress, a shadowy figure named Jean Lupin emerges, claiming that the true Kogorou Akechi exists elsewhere and that the Fiend is an impostor. The final arc deals with a series of murders targeting women with tear-filled eyes, committed by a killer known as the Spider Man. Kobayashi is abducted, and the Fiend’s pursuit of the truth leads to a confrontation with Jean Lupin, who has been fabricating the Spider Man as a hoax. In the climactic showdown, the Fiend faces the “real” Akechi and the deepest darkness of his own nature, ultimately deciding the fate of his dual identity.