After his girlfriend Yuuko breaks up with him, Miyu Samura's luck is at an all-time low. But one day several months later, she reappears and gives Miyu a toy poodle she supposedly cannot take care of. Miyu's knowledge of dogs is nonexistent, and the puppy's rowdiness leaves him in danger of being evicted from his own home. When Miyu decides to take the puppy on a walk, he has a chance encounter with professional dog trainer Shinichirou Niwa and Niwa's well-behaved standard poodle Ourson. In just a matter of minutes, Niwa not only corrects the attitude of Miyu's dog, but also gives Miyu himself stern advice on how to treat his pet properly. Amazed by Niwa's techniques, Miyu decides to quit his job in order to become the trainer's assistant; however, Niwa is not as skillful in dealing with humans as he is with their canine companions. Even so, Miyu refuses to give up on Niwa and his failing business.

In the town of Kurouzu-cho, Kirie Goshima lives a fairly normal life with her family. As she walks to the train station one day to meet her boyfriend, Shuuichi Saito, she sees his father staring at a snail shell in an alley. Thinking nothing of it, she mentions the incident to Shuuichi, who says that his father has been acting weird lately. Shuuichi reveals his rising desire to leave the town with Kirie, saying that the town is infected with spirals. But his father's obsession with the shape soon proves deadly, beginning a chain of horrific and unexplainable events that causes the residents of Kurouzu-cho to spiral into madness.