Arcana features a humorous plot starring two detectives, Murakami and Nakabayashi, and an unidentified mysterious girl who possesses the power to see spirits. As the story unfolds, Murakami and Nakabayashi have been assigned to a serial bombing case that specifically targeted elementary school kids. Neither of them have any idea of what to go about. Their only source: the prophecy of a patient suffering from amnesia and currently undergoing rehabilitation at a local mental clinic. The detectives try to uncover the secret behind the mystery girl as job reassignments and supernatural occurrences occur more and more frequently.