In an alternative feudal Japan, a strange disease that only affects males caused a massive population reduction, leaving females to fill the vacant jobs, therefore changing the social structure. Now, 80 years after the initial outbreak with a 1:4 male:female ratio, Japan is a completely matriarchal society. Females hold all important political positions, and males are their consorts. Only the most powerful female—head of Tokugawa shogunate—may keep a harem of handsome and unproductive males, known as "Ōoku".