In the rural, mountain-surrounded town of Akada, high school student Shiori Nakagawa has stopped attending classes due to persistent bullying. Shiori’s life changes with the sudden arrival of Mei Satou, a girl who comes to live in Shiori's home as a cohabitant or renter. While Mei is reticent with everyone else, she reveals her true purpose only to Shiori: to kill the seven people responsible for her older sister Akari’s death.

Three years prior, Akari was driven to suicide by the horrific bullying she endured in this same town. To achieve her revenge, Mei plans to perform the blood-soaked ritual known as Okakashitsutsumi, a tradition of the local Akada Shrine. The ritual allows her to embody the giant serpent god Okakashisama, granting her the power to execute her plan. On the night of the summer solstice, Mei successfully completes the ritual.

From that point forward, Mei, with the power of the god and aided by Shiori, begins systematically killing those involved in her sister’s death, one by one. As they spend their days in a cycle of vengeance and escape, the two protagonists develop romantic feelings for each other. However, Mei harbors a devastating secret from Shiori: anyone who performs the Okakashitsutsumi ritual must ultimately offer their own soul to the god in exchange for its power, making Mei’s own death inevitable.