The story begins on Hell Island, the only remaining penal island in the modern era, where the legendary yakuza convict Rikido Omasa is granted parole after serving 30 years. He is told by Inspector Jirocho Shimizu of the Ominami Police Station—the man who once arrested him—that this is a case of “using poison to fight poison”: a prisoner, a “deadly poison,” will become a detective to suppress the “poison” of crime that infests modern society…