Jae-hyuk is a pharmaceutical worker for a medical supplies company. 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 hunger and thirst. Hence, they jump into the river to combat dehydration. While the authorities work to find a cure, Jae-hyuk struggles to save his family when they also exhibits similar symptoms.