Tatsumi Sekiguchi reads part of a transcript, titled "The Woman inside the Box" by Shunkō Kubo; and imagines himself as the story's protagonist. In it, he is traveling to his grandmother's funeral on a train with another man, who possessed a box containing the live head of a girl. In the main storyline, Kanako Yuzuki chooses classmate Yoriko Kusumoto to be her friend. Yoriko's mother, Kimie, believes Yoriko has been influenced by a mōryō, an evil spirit, when Yoriko begins taking nighttime strolls with Kanako. Kanako and Yoriko plan to go to Lake Sagami over summer break. On the night they leave, Yoriko witnesses Kanako crying, as well as a pimple on the back of her neck; both points perplex Yoriko. Soon after, Kanako is hit by the train on which detective Shutarō Kiba is traveling.

Kiba thinks about the events of the last few days. In a non-linear flashback, Fukumoto escorts the transfer of Kanako to Mimasaka's hospital, a virtually-windowless box-like building in the middle of a forest. This facility is staffed by only two doctors, Mimasaka himself and Tarō Suzaki, as well as a maintenance engineer. Meanwhile, Kimie has become even more convinced that Yoriko is possessed. She brings Hyōei Terada to her house. He performs an exorcism of the house, and tells her that she can purify herself by giving him her tainted material wealth. Yoriko tells Kiba that Kanako was pushed by a man wearing gloves. Kanako disappears from the hospital, and Kiba later discovers Yōko with a ransom note in her hand.

Sekiguchi reads more of "The Woman inside the Box", imagining himself as the protagonist. In it, he dismembers girls and wonders why he is unable to keep their heads alive. He resolves to meet the doctor that kept the girl alive in the box. In the main storyline, Kiba continues his flashback. Yōko asks Kiba to investigate, despite the Kanagawa police having jurisdiction. Later, Suzaki is found murdered, and Amemiya disappears. Back in the present, Sekiguchi's editor asks him to review a manuscript from Shunkō Kubo. A gloved man dismembers a prostitute. After Kiba is suspended from the Tokyo police, Bunzō Aoki asks him for help on the Musashino dismemberment case and Kanako's kidnapping. Kiba investigates Yōko and discovers that, during her film career, she was stalked by someone matching Suzaki's description. Kiba also investigates Mimasaka and discovers that he researched the creation of artificial, immortal soldiers during the war.