In a remote mountain village in South Korea, although life is not affluent, there has been a long-lasting peace, and people seem unaware of approaching danger. Everything changed one evening when a sudden murder occurred in the village, shattering its tranquility. Chonggu, who serves as a police officer in the village, and his peers quickly rushed to the scene to investigate. He initially deduced it to be a heinous murder, but locals privately speculated that a Japanese man, who had mysteriously appeared in the village's outskirts, was the prime suspect. This Japanese man was suspected of practicing some form of sorcery, cursing people to death through supernatural means.

One night, numerous tiny organisms, ominously dubbed "parasites," drifted onto the sea surface. Their target was to parasitize the human brain, instigating people to turn against each other in a frenzy of violence.