In the town of Kurouzu-cho, Kirie Goshima enjoys a relatively ordinary life with her family. One day, while heading to the train station to meet her boyfriend, Shuuichi Saito, she notices his father fixated on a snail shell in an alley. She thinks little of it and mentions it to Shuuichi, who shares that his father has been acting strangely lately. Shuuichi expresses his growing urge to leave the town with Kirie, claiming it is cursed by spirals.
However, his father's obsession with this shape soon turns deadly, triggering a series of horrifying and inexplicable events that plunge the residents of Kurouzu-cho into madness.