Just before dawn, gruesomely skeletal bodies surface on the Han River. The cause of death is determined to be a mutant parasite-a hairworm--that brainwashes its hosts, ultimately making them jump into bodies of water and to their death. Jae-hyuk, a salesman for a pharmaceutical firm, faces an enormous terror: his entire family has become host to the fatal parasite. But while he searches for the cure to save his family, the parasite becomes a nationwide epidemic. The government takes action and builds a camp for the patients, experiencing only a series of failures in their attempts to cure them of the horrifying disease.