A young couple looking to buy a house asks a friend who is an author specializing in the occult to examine the property's floor plans. From the moment he sees the blueprints, the writer feels something is deeply wrong, sensing that the house is hiding a secret space within its walls. He brings the plans to Kurihara, an architect friend, who starts pointing out a disturbing series of anomalies. They identify a children's bedroom placed in the center of the floor with no windows, hidden behind a hallway and two doors. Elsewhere, the blueprints seem to indicate a sealed room without any visible access point, and what appears to be a secret passage leading to the garage.

The occult writer and Kurihara work together unraveling the mystery of the house's true purpose. When they overlay the different levels of the blueprints, they discover that the corners of the hidden children's room and a second-floor bathroom perfectly align with the unmarked, concealed space on the ground floor. As they theorize about why the house was built this way, their speculation grows increasingly grim, suggesting the house's very architecture was designed for murder, such as creating a space where an unseen person could move through the walls to attack residents. The writer finds himself drawn into an immersive and obsessive investigation where the boundary between reality and frightening theory becomes increasingly blurred.

The narrator, an unnamed freelance writer who specializes in occult and supernatural mysteries, serves as the protagonist whose perspective guides the investigation. His practical and logical architect friend, Kurihara, helps analyze the structural anomalies that a normal person would miss. The setting is initially an ordinary, quiet residential neighborhood in or near Tokyo, but the narrative center is the house itself, which becomes a character defined by its unsettling floor plan.

The initial narrative arc involves the discovery of the house's bizarre blueprint and the informed but speculative analysis of its strange features. As the writer and Kurihara dig deeper, they transition from architectural curiosity to dark theorizing, suspecting a child was hidden in the secret spaces to carry out attacks. Later developments involve the writer becoming obsessed with the case, haunted by invasive thoughts and nightmares as he tries to discover the truth of what happened within those walls.