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Welcome to the classes of horror! I will be your guide to a world that lurks under everyday life. For these lessons, you don't need text books or notebooks. If you're prepared for the worst...
Doppo Kunikida is an idealist and a strait-laced detective at the Armed Detective Agency, an organization that takes on dangerous jobs that even the police won't handle. Everything in his life is progressing as planned until one day he's ordered to watch over the new guy, Osamu Dazai, a suspicious fellow of unknown origin who is obsessed with the thought of suicide. While investigating an unusual case involving a so-called haunted building, they find several of the missing people from another investigation. However, before they can find out how these cases connect, they are attacked by the mafia's Ryunosuke Akutagawa, sworn enemy of the agency! This is the beginning of a hopelessly inseparable relationship between two detectives. A battle of Abilities erupts in the shadows of Yokohama!
The narrator, Francis Wayland Thurston, recounts his discovery of notes left behind by his grand-uncle, Brown University linguistic professor George Gammell Angell after his death in the winter of 1926–27. Among the notes is a small bas-relief sculpture of a scaly creature which yields "simultaneous pictures of an octopus, a dragon, and a human caricature." The sculptor, a Rhode Island art student named Henry Anthony Wilcox, based the work on delirious dreams of "great Cyclopean cities of titan blocks and sky-flung monoliths." Frequent references to Cthulhu and R'lyeh are found in Wilcox's papers. Angell also discovers reports of mass hysteria around the world.