A male corpse is found in the mountains, and Ibuki and Shima rush to the scene. Due to the characteristic wounds on the body and the letters left behind, the possibility of unsolved serial murder emerges, and the investigation of the case is under the jurisdiction of one division. Ibuki and Shima, who participated in the investigation meeting, are immediately assisted by a detective, Kariya. Ibuki and Shima meet clinical laboratory technician Sakamoto at the Unnatural Death Research Institute, commonly known as "UDI Lab," who accompanied Kariya. After that, Ibuki steals the investigation materials of this case and learns that the detective in charge of the first case of a man with multiple criminal records was his teacher, Gamagori. Ibuki and Shima follow the criminal, relying on information from Gamagori...