The girl dedicated her life to avenging the seven people who killed her sister. Akada Town, a rural area surrounded by mountains on all sides. Nakagawa Shiori, a high school student in this town, had stopped attending school due to bullying. One day, Mei Satou comes to live with Shiori as a cohabitant. Mei, who doesn't open her heart to anyone, reveals her purpose for coming to this town only to Shiori. “I want to kill the seven people who drove my sister to her death.” Three years ago, Mei's sister, Akari, was subjected to horrific bullying in this town and took her own life. Mei has returned to exact revenge. She plans to perform the blood-soaked ritual 'Okakashitsutsumi,' handed down at Akada Shrine, to embody the giant serpent god 'Okakashisama' and kill the seven people involved. On the night of the summer solstice, Mei successfully performs the ritual. From that point on, with the power of the god and together with Shiori, Mei begins to kill those involved in her sister's death, one by one each day. Amid their days of vengeance and escape, the two eventually develop romantic feelings for each other. However, Mei keeps one secret from Shiori: anyone who performs the 'Okakashitsutsumi' must ultimately offer their soul to the god, meaning Mei's own death is inevitable.