By day, Saya Kisaragi is a peaceful schoolgirl; by night, she is a fearsome monster slayer, leading a double life. Armed with a ceremonial sword her father gave her for sacred duties, she defeats every monster that dares to threaten her quiet little village. However, Saya's reality and everything she believes to be true are soon tested when she overhears the monsters speaking of a broken covenant—something she knows nothing about. Then, unexpectedly, a strange dog appears and asks her to whom she promised to protect the village, curious about what would happen if she were to break that promise. Tormented by unexplainable visions and her world unraveling around her, we follow Saya as she struggles to find a way to the truth in a village where nothing is as it seems.