Jae-hyuk (Kim Myung-min) is a former professor with a doctorate in biochemistry who is currently working as a pharmaceutical sales representative after losing his life savings and his job due to a bad investment he made in the stock market from advice he received from his younger brother. When a series of dead bodies are found floating in the Han River, the public is shocked to discover that the deaths are related to a fatal outbreak of virus-infected mutant parasitic horsehair worms, called Yeongasi, that can control the human brain. Those infected show symptoms of increased hunger without appropriate weight gain and excessive thirst when the worms are mature and ready to reproduce. Hence, they jump into the river to allow the worms to come out of the body. While the authorities work to find a cure, Jae-hyuk and his brother Jae-pil (Kim Dong-wan), a detective agonized with guilt for squandering Jae-hyuk’s money in the stock market, struggle to save Jae-hyuk’s family when they also exhibit similar symptoms.