Once every hundred years, a village must offer a sacrifice to appease the gods or face destruction.
Kouzuki Sayo comes from a family of priestesses with special powers. As the day of sacrifice approaches, Sayo's older sister, Mahiru, volunteers to take her place. Sayo protests, but is imprisoned while Mahiru disappears as the sacrifice. To break this tradition and save her sister, Sayo takes up a sword and ventures into the lair of the monsters responsible for the sacrifices.