On Holy Saturday, outside the small town of Pottsville, Idaho, a terrified teenaged boy runs from a nuclear waste dump and steals a car from a wrecking yard. As he speeds down the road, the long, slimy arm of a monster-like creature punctures the roof and pulls off the boy’s head. Later that evening, as police detective Mortimer “Mort” Lutz and a tow truck driver examine the abandoned car for evidence of its owner, the trunk opens and the driver vanishes. Mort is puzzled by the driver’s disappearance but returns to the station to write his report. At the police station, Mort watches a television interview with Dr. Garson Jones, a chemist who advises the state of Idaho on environmental safety. Dr. Jones insists that leakage from the nuclear waste dumpsite is not contaminating Pottsville’s drinking water, though he declines to offer any evidence to support his statement. Meanwhile, the monster terrorizes a drive-in theater, killing three people. Mort is summoned to the theater, where he examines an abandoned car splattered with green slime, and a small hole in the ground next to it. An eyewitness claims that his friend was pulled from their car by “a guy in a monster suit,” but Mort assumes the account to be a drug-induced hallucination. He returns home shortly before dawn to find the monster lying in wait and runs from the house, escaping from the creature as the sun rises. Later, Mort tells Mayor Gordon Lane about the monster, but the mayor refuses to publicize the danger, as it might damage the town’s status as the world’s potato capital. Jones arrives and tells Mort and Mayor Lane that he will stop the creature without causing a disturbance. At a diner on Main Street, Mort fears for the safety of Laurie, one of the waitresses, and offers to take her home when her shift ends at 7:30 p.m. At the police station, a television news reporter questions Dr. Jones about a radioactive substance discovered in Pottsville. He informs her that he will camp at the site and perform an in-depth investigation, but continues to deny any danger. Mort returns home and falls asleep, haunted by nightmares about the monster. He is awakened by a phone call from Dr. Jones, summoning him to the dumpsite. Realizing that he has overslept, Mort stops by the diner to pick up Laurie, who has been waiting four hours for him. When they discover the monster hiding in Laurie’s car, they return to the diner and lock the doors. The monster gains entrance, but Laurie locks it inside the meat freezer. Mort calls Mayor Lane and asks for reinforcements, but the mayor is intent on keeping the monster secret and arrives at the diner alone. When Mort opens the freezer, the monster is nowhere to be seen, prompting Mayor Lane to scold the detective for wasting his time. Outside, a storm gathers and an eccentric woman named Marge Smith calls for her missing son, Michael. At the mayor’s home, his wife, Virginia, investigates a noise in the garage and suddenly finds the monster’s snake-like tongue wrapped around her neck, while her husband sits in his car drinking whiskey. Meanwhile, Dr. Jones tells Mort and Laurie about the network of tunnels created by the monster, which allows it to travel through the town unseen. He has determined that light is harmful to the creature, explaining why it only appears at night. At that moment, the monster ensnares the doctor’s ankle. Laurie cuts Dr. Jones loose with an axe and they drive to the police station to arm themselves. Despite Laurie’s insistence on joining the fight, Mort locks her in a jail cell to keep her safe, leaving instructions for the next shift of officers to release her. When Laurie sees Marge Smith standing outside her home across the street, an officer explains that “she’s been cooking on another planet” since her son, Michael, disappeared at the dumpsite. Laurie and the officer notice puddles of green slime dotting the front porch, which Marge attributes to Michael. At the dumpsite, Dr. Jones describes the monster as a highly intelligent but “completely psychotic” genetic mutation created by overexposure to radioactivity. He fears that similar creatures may exist in other dumpsites around the country. The men rig a pickup truck with an explosive charge, lure the monster into it, and leap from the cab before the charge detonates. Believing they have killed the monster, the men enter a chemical warehouse to call for reinforcements, but the phone is out of order. As they leave, the monster again grabs Dr. Jones by the ankle and drags him away. When Mort discovers the doctor’s dismembered body, he arms himself with an axe and goes on the offensive. However, Mort is no match for the creature and he sustains several injuries before it backs him into a corner. Finding a supply of sulfuric acid close at hand, Mort douses the creature with it while shining a flashlight in its face. He then hacks the monster with his axe, causing the carcass to explode. Mort emerges from the warehouse as a radio announcer reports Dr. Jones’s assurance that there is no danger from the nuclear dumpsite, and a slimy hand reaches up from under the ground.