Akari Fusegi is a civil servant from the Ministry of Health, Labor and Welfare. One day, she encounters a mysterious death of a man, who turns out to be an "okinaga", a person who is immortal and will not die unless killed in a specific way. Akari is later assigned to work together with Kai Yukimura, an okinaga who looks 18 but is actually 88 years old. Together, they will investigate a series of murders by the "Ram Killer" that have been occurring every Christmas in the year of the Ram over a period of 70 years.
