On a rural farm in Tellico Plains, Tennessee, a teenage boy Zack navigates life with his mother Frances Crane, his younger sister Alice, stern stepfather Nathan Crane, and blunt stepbrother Cyrus Crane. One night Frances sneaks out and has sex with Mike, a farm-hand who lives in a nearby shack. Then a large meteorite crashes on the property, emitting an eerie glow. The next morning, Dr. Alan J. Forbes visits the crash site to study the object, a thick-shelled sphere from which a noxious liquid seeps. The meteorite dissolves into glowing gelatinous matter that seeps into the soil. Forbes wants to contact authorities, but Charlie Davidson—a local realtor and head of the chamber of commerce—advocates keeping quiet to avoid jeopardizing a TVA reservoir project. Esther Forbes, Forbes's bored wife, also pushes to keep things beneath the radar to protect their home’s value.
Soon the strange liquid begins to affect the farm in unsettling ways: the well water becomes cloudy and tastes odd; fruit and vegetables grow enormous but are rotten inside; livestock behave violently and show signs of infection. Alice is attacked by infected chickens, and Cyrus is nearly killed by a vicious horse. Frances develops large boils that grotesquely distort her features and drive her toward violence, harming herself and her family.
To understand the phenomenon, Forbes secretly takes a sample and has it analyzed at a nearby lab; the test reveals a strange, unknown element that alters water’s metabolic properties. Carl Willis, a TVA representative surveying the area, enters the Cranes’ house to fetch a glass of water and is attacked by Frances, now infected. Davidson and Esther arrive at the farm seeking the doctor but are attacked by infected dogs; Esther is mauled to death and Davidson hides in the cellar, only to be killed and apparently devoured by Frances, who had been locked away earlier by Nathan. As Nathan and Cyrus examine infected cows, the cows’ decay reveals maggots and worms inside; the cows explode, splattering Nathan and Cyrus with insects.
By this point Nathan and Cyrus are infected and slipping toward madness. Forbes returns, hoping to rescue Zack and Alice, but Nathan murders him and locks the door. Cyrus attacks Alice, but Zack fights him off and hides his sister in a closet. Nathan corners Zack and is about to kill him when Carl Willis arrives and shoots Nathan, just as the ground begins to glow and the house starts to fall apart. Willis frees Zack and Alice as the structure collapses, and the deaths of Nathan and Cyrus come beneath the debris. Willis drives away from the farm with Zack and Alice.
Months later, a heavily-bandaged Willis lies in a hospital bed, infected more slowly because he drank only a small amount of the farm’s water. He watches a news report on authorities promising the blight will be eradicated, while in the surrounding countryside the ground and trees begin to heave at night, revealing more of the glowing alien liquid creeping across the surface. A looming sense remains that the hostile mutagenic entity is still alive and poised to complete its invasion of Earth.
