In Kurouzu-cho, Kirie Goshima leads a relatively ordinary life with her family. One day, while walking to the train station to meet her boyfriend, Shuuichi Saito, she spots his father in an alleyway, fixated on a snail shell. Dismissing it as unimportant, she tells Shuuichi about the encounter. He remarks that his father has been behaving strangely recently. Shuuichi then expresses his growing wish to flee the town with Kirie, claiming that Kurouzu-cho is infected with spirals. However, his father's fixation on the shape soon turns fatal, triggering a sequence of terrifying and inexplicable events that drive the residents of Kurouzu-cho into madness.