The title involves the twin Amatsu sisters, Ai (天津亜衣) and Mai (天津麻衣), students at the Tenjin Girls High School who become virgin priestesses that must watch over the boy, Onimaru (鬼麿), a distant relative of theirs. It becomes his choice to make the world a hellhole of debauchery or to keep as it is. Onimaru (鬼麿) is the descendant of a line of men whose founder was betrayed by the Japanese government, and whose spirit seeks revenge by making Onimaru a sexually depraved demon king. The founder is the younger brother to the twin sisters. The three of them are the result of a union between a fisherman and an Angel (Tennyo) of the Buddhist tradition.
As in traditional Japanese folklore, the man acquired his angelic wife by stealing her angelic robes while she bathed, so that she could not return to heaven, and blackmailing her to marry him. (This is also the basis for the anime Ayashi no Ceres/Ceres: Celestial Legend). The folklore originates in ancient China, known as "Tian Xian Pei", the love legend between an angelic girl and an ordinary cowherd boy. The two original girls were the founders of a temple and order that rein in the descendants of the betrayed boy, their younger brother. The twin sisters of the anime, older sister Ai (with a long ponytail) and younger sister Mai (with short hair), descendants of that Angel and her two girls, fight the forces of sexual perversion, in the demonic minions of the founding son, with the help of grandmother Amatsu Genjuu (天津幻舟), and Onimaru's guardian Dekonobu (木偶の坊).
