The action takes place in Japan during the Edo period. An unknown disease, called "red smallpox," only affects men. After several decades since the first outbreak, the number of women has significantly increased, and a matriarchy has emerged in the country. Women now possess all the skills and abilities traditionally associated with men. They protect men from the threat of extinction, and the most attractive men are sent to the inner chambers of the most powerful woman in Japan, the head of the Tokugawa shogunate.