Ichitarou, the sickly heir to a large apothecary in the Edo period, has spent most of his life confined to his room because of his weak constitution. Rumors about his condition spread unpleasantly around him, so his grandfather prays for spirits to protect the boy. In response, two guardians—Sasuke and Nikichi—are sent to watch over Ichitarou for his entire life and preserve his health.
As Ichitarou grows older, he starts leaving his room more often, wanting to be useful despite others urging him to rest. One day, he slips out alone without his guardians and remains outside until dark. On his way home, a bleeding man suddenly chases him. Ichitarou manages to escape using his quick thinking and stumbles upon the scene of a brutal murder.
The next day, he hears about the killing, but the story seems strange—the victim's fatal wound differs from what Ichitarou witnessed. With the murderer still at large, Ichitarou begins gathering more details about the peculiar case from both humans and spirits, and these clues may help him solve the mystery.