The Ukishima sanctuary is located in a picturesque old town on the edge of a lake. Saya Kisaragi, a young priestess of the temple lives there with her father, Tadayoshi. During the day, she lives the life of a normal high school student at the Sanbar academy. But at night, she hunts the "Ancient", creatures possessing supernatural physical abilities who prey humans and which alone she has the ability to defeat.