The Waters of Crystal Lake appear serene and beautiful in the fall sunshine of 1979. Yet something is silently glowing under its surface that could wreak havoc in the fictitious town of Greenwood, which depends on the lake for its water. Meanwhile, twenty-eight-year-old divorcee, Debra Chandler, is working in a Nashville research lab for the brilliant, but difficult Dr. Joseph Steiner, an authority in DNA research. Soon her quiet and boring world will be turned upside down when a new boyfriend, medical resident Dr. Jim Tarkington, asks her to accompany him to Greenwood to investigate a possible epidemic. Coincidently, this is the same town where her favorite cousin Elizabeth Chandler lives. Elizabeth works as an office nurse for a doctor in family practice. Shortly they and the rest of the medical community will be swept up in a disaster of epic proportions. Jim and Debra arrive at Greenwood Memorial planning a short visit to collect data for his research paper. There they find a chaotic scene with the hospital overflowing with deadly ill patients suffering from an unknown disease. The medical personnel have no idea what is causing the illness as they frantically attempt to treat the sick. After interviewing patients and arranging to collect samples from them, Debra and Jim, exhausted, spend the night at Elizabeth's home. The next day, acting on a hunch, they also obtain samples from the Greenwood Water Company before returning home. Back in Nashville, after careful laboratory analysis of the samples, they discover an unbelievable reason for the epidemic. This information will later force the decision-makers to admit a madman is loose in the city.