Kumagai serves as a guard at the Shōnan Special Reformatory, temporarily replacing the sadistic senior guard Ishihara after Ishihara is placed on house arrest due to a facility fire. Demonstrating compassion toward the inmates, Kumagai reveals Ishihara's schemes targeting their senior, Rokurouta Sakuragi, to the boys in Cell Block 2, Room 6. Recognizing the immediate threat to Sakuragi's life from Ishihara and Dr. Sasaki's plot, Kumagai secretly delivers food to sustain Sakuragi during starvation conditions in solitary confinement.

Later, Kumagai discovers Sakuragi undergoing water torture orchestrated by Ishihara. While attempting to intervene and aid Sakuragi, Kumagai is ambushed by Ishihara. Kumagai dies shortly after this confrontation under circumstances orchestrated by Ishihara. Ishihara claims the death was an accidental drowning, framing it as unrelated, but this covers up his deliberate act of murder to eliminate Kumagai for his interference and knowledge of the conspiracy. Kumagai's death significantly escalates tensions within the reformatory, underscoring the extreme corruption and danger posed by Ishihara and Dr. Sasaki.