Saya Kisaragi is a seemingly ordinary high school girl living in a quiet, rural town. She serves as the shrine maiden at her family's shrine and leads a peaceful life, surrounded by friends and her father, Tadayoshi Kisaragi. However, Saya has a hidden duty: she is a skilled swordswoman tasked with protecting the town from monstrous creatures known as Elders. These beings emerge at night, preying on humans, and Saya battles them with her katana, guided by a mysterious talking dog named Tadayoshi, who shares her father's name.

As the series progresses, Saya begins to notice inconsistencies in her surroundings. Strange occurrences, such as recurring dreams and cryptic warnings from Tadayoshi, suggest that her reality may not be as it seems. The townspeople exhibit odd behavior, and the Elders' attacks grow increasingly coordinated and personal. Saya's memories are fragmented, and she struggles to reconcile her dual roles as a high school student and a warrior. Her relationships with her classmates, particularly Fumito Nanahara, a boy who seems to know more than he lets on, add layers of tension and intrigue.

The narrative takes a darker turn as Saya uncovers the truth about her existence. The town is revealed to be an elaborate experiment orchestrated by Fumito, who is part of a secretive organization. The Elders are not random monsters but constructs designed to test Saya's abilities and mental resilience. The experiment aims to awaken her true nature as a powerful being capable of immense destruction. As Saya grapples with this revelation, her world unravels, leading to a climactic confrontation that exposes the full extent of the manipulation she has endured.

The series explores themes of identity, memory, and the nature of reality, blending action, horror, and psychological drama. Saya's journey from a seemingly ordinary girl to a figure of immense power is marked by violence, betrayal, and self-discovery. The supporting cast, including her classmates and the enigmatic Tadayoshi, play crucial roles in shaping her understanding of the world and her place within it. The story's intricate plot and gradual unraveling of mysteries keep the narrative engaging, culminating in a finale that leaves a lasting impact.