In the town of Kurouzu-cho, Kirie Goshima leads a fairly ordinary life with her family. One day, while walking to the train station to meet her boyfriend, Shuuichi Saito, she notices his father staring at a snail shell in an alley. Dismissing it as unimportant, she tells Shuuichi about the incident, and he remarks that his father has been acting strangely lately. Shuuichi then expresses his growing wish to leave the town with Kirie, claiming that the town is infected with spirals. However, his father's obsession with the shape soon proves fatal, triggering a chain of horrific and unexplainable events that drives the residents of Kurouzu-cho into madness.