Jae-hyuk, an employee at a pharmaceutical company, finds himself in a dire situation when numerous corpses are discovered in the Han River. The community is alarmed to learn that these deaths are linked to a deadly virus outbreak caused by mutant parasitic horsehair worms known as ‘Yeongasi’, which have the ability to manipulate human brains. Infected individuals experience intense hunger and thirst, driving them to leap into the river in an attempt to quench their dehydration. As officials race to develop a cure, Jae-hyuk faces a personal battle to protect his family, who begin to show similar symptoms.