The title involves twin sisters, Amatsu Ai (天津亜衣) and Amatsu Mai (天津麻衣), 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 keeping as it is. Onimaru (鬼麿) is the descendant of a line of men, of whom the founder was betrayed by the Japanese government, and whose spirit wants revenge through his descendant, by making Onimaru a sex depraved demon king. The founder is the younger brother to two twin sisters. The three of them are the result of a union between a fisherman and an Angel (Tennyo) of the Buddhist tradition.
The man got his angelic wife through the traditional Japanese folklore means, 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 are the founders of a temple and order that rein in the descendants of the betrayed boy, their younger brother.
The two twin sisters of the anime, older sister Ai (with long hair) 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).
Character is error.