Fusemori Souma departs from his long-time residence deep in the mountains and sets foot in Ouma—one of Japan's major cities.
In this urban setting, there is a religious group called the "Mayoi" (Exorcist) clan, whose purpose is to worship the gods and eliminate the "Yama" (Night Demons) that disrupt the harmony of the world. Due to an age-old pact between this clan—specifically, the Mikosono family—and the Fusemori family, Souma encounters a young girl about his age. Her name is Mikosono Himeka, a girl burdened with an extraordinary destiny.
Draped in a sacred garb and wielding a divine sword, Himeka protects the town as "Sennagi" (warrior priestess), the vanquisher of Night Demons. However, recent times have seen an increase in Night Demon-related incidents around Ouma, and rumors circulate of particularly powerful Night Demons on the loose.
Thus, following the ancient pact between the Mikosono and Fusemori families, they decided to conduct a particular "divine ritual" to enhance the power of the Sennagi.
The ritual to be performed is unorthodox and heretical, in which the maiden is subjected to various forms of humiliation to channel divine power into her. What is offered up is the pure heart of a virgin.
—Himeka is defiled, and by offering her heart, her Sennagi powers awaken.
For the protection of humankind and for her to become stronger as a Sennagi, Himeka sacrifices her body and heart. The role given to Souma is to support Himeka in various ways and to serve as the vessel for the god during the "divine ritual," thereby defiling her as the executor of the sinful practices.
—All of this, to defeat the Night Demons.
—Her body, for the peace of all.
Souma and Himeka consistently engage in this lustful ritual and devote themselves to the ongoing fight against the Night Demons.
[Translated and edited from the official website]