The protagonist is Mitsuru Moterugi, a fairly unpopular guy who visits a shrine in the hope that it will improve his luck with women. However, he accidentally destroys an altar which is said to hold evil spirits that would otherwise threaten the mortal world. When one of those spirits seemingly possesses one of the shrine maidens, Shiori Toyofuta, Mitsuru learns that the only way to exorcise the spirit is to imbue her with the spiritual power contained within his semen. After taking Shiori's virginity, Mitsuru sets out to vanquish the spirits who have possessed the other two shrine maidens, Shiori's little sister Kaori and Mei Abenose.

[From MobyGames]