Fushiki Akari (Matsui Airi) of the Ministry of Health, Labor and Welfare encountered a bizarre case while interning at a health center. The murdered men have a special physique, commonly known as the "Isaga", and have the ability to never grow old or die. They can only be killed by a specific method. A few days later, Akari was assigned to the Night Health Management Section (abbreviated as Night Guard Management), which is responsible for managing the "Isaga". She went there with her boss, Kuboson Mikiya (Mitsui Ken). In the private library, the Ansashi Library, they met Yukimura Kui (Kamiyama Tomohiro), who looks only 18 years old, but is actually 88 years old. Kui has been tracking down the serial murder case that has lasted for 60 years and occurs every 12 years of the Year of the Sheep. Akari became Kui's liaison and investigated the case related to the "Isaga" with him.