Saya Kisaragi has a double existence as a peaceful schoolgirl by day and a ferocious monster killer at night. She vanquishes any creature that dares to attack her peaceful small hamlet, armed with a ceremonial sword given to her by her father for holy purposes. But, all too soon, Saya's reality and all she thinks to be true are put to the test when she overhears the monsters discussing a broken covenant—something she is unaware of. Then, suddenly, a strange dog approaches; it inquires as to whom she vowed to defend the community, inquiring as to what would happen if she broke that commitment. We go alongside Saya as she struggles to find a route to the truth, tormented by strange visions and her world disintegrating around her.