Naoto Tsugimura, head of customer service at a major food and beverage company, receives a strange complaint from an angry caller who claims to have found insects in his mineral water. Tsugimura is unable to appease him, and soon finds that this mysterious claimer is beginning to infiltrate his life, harassing and stalking him, but always out of Tsugimura's sight. Nothing Tsugimura does - apologizing, fighting back - seems to make any difference. Meanwhile the claimer's rage continues to escalate, until finally reaching a shocking conclusion.