Ichitarō is the heir to Nagasakiyama, a prominent commercial establishment in Edo-period Nihonbashi. Physically frail since birth, his condition confined him indoors for much of his early life and necessitates constant protection from supernatural entities accompanying him.

From age five, Ichitarō gained the ability to perceive spirits, forming friendships with supernatural beings assigned as his guardians. Key among these are Sasuke, an inugami (dog spirit), and another spirit known as both Nikichi and Jinkichi, a hakutaku (mythical creature). These companions serve as his primary protectors and aides due to his vulnerability.

A pivotal moment occurs when Ichitarō ventures outside without permission one night and witnesses a murder firsthand. This event coincides with a series of unexplained killings spreading through Edo. Prompted by his observation, he undertakes investigations into these mysteries. His spirit companions actively support these efforts, leveraging their supernatural capabilities to assist him in uncovering the crimes' truths.