Shutaro Kiba operates as a Tokyo police detective in 1952. His duties converge with a pivotal incident when he observes Kanako Yuzuki struck by the train he occupies. He spearheads the initial investigation, engaging with witness Yoriko Kusumoto, who asserts a gloved man pushed Kanako onto the tracks.

Kiba bears psychological scars from his World War II combat service. Persistent nightmares plague him, depicting wartime horrors like fallen comrades and monstrous tanks crushing soldiers. This trauma disrupts his sleep and fuels an irritable disposition. Though functional in his detective role, the trauma intermittently clouds his judgment during investigations. His wartime service also establishes a prior connection to Tatsumi Sekiguchi, identified as his former senior officer.

Professionally relentless, Kiba pursues Kanako's disappearance after her transfer to Kōshirō Mimasaka's research hospital. He confronts Mimasaka directly, accusing him of involvement in local dismemberment murders and demanding Kanako's whereabouts. This encounter underscores Kiba's intense focus and suspicion toward the ethically questionable researcher. His investigation ultimately draws him to Mimasaka's facility alongside other key figures as multiple plot threads converge.

During the facility's climactic events, after the deaths of Mimasaka and Shunkō Kubo, Kiba executes his duty as a law enforcement officer by arresting Yōko Yuzuki for Kubo's murder.