In the town of Pottsville, Idaho, residents begin to vanish, with young Michael Smith—the son of Marge Smith—the first to disappear. A decapitated escapee and three drive-in theater patrons are found murdered, and at each scene a slick green slime lingering in the outskirts of the tragedy points to something unnatural at work.
To uncover the truth without risking the town’s potato-based economy, Mayor Gordon Lane hires Garson Jones, a seasoned chemical safety engineer, to lead the investigation. At the same time, Detective Mortimer Lutz quietly pursues his own line of inquiry, convinced that something terrible has indeed taken hold of Pottsville.
As the disappearances mount, a recurring puddle of green slime is discovered at every new site. Lutz grows suspicious that Jones knows more than he’s admitting, a doubt that hardens after he is attacked by a monstrous presence in his own home and narrowly escapes.
One night, while escorting his waitress girlfriend Laurie home, Lutz and Laurie are suddenly assaulted by the same grotesque creature. They barricade themselves inside a diner and seal the place off in a freezer, then reach out to the mayor. When Mayor Gordon Lane arrives, the creature has vanished, leaving behind only the slime as evidence.
With tensions rising, Lutz presses Jones again, who finally acknowledges that a highly radioactive creature is responsible. He reveals that Pottsville sits atop one of 2,000 nuclear dump sites in the United States, and the mutant—believed to be Michael Smith—arose from repeated exposure to the site’s radioactive materials. The creature is depicted as intelligent yet psychotic, and it is vulnerable to light, becoming inactive during the day.
Armed with shotguns, Jones and Lutz track the creature to an abandoned warehouse, where the hungry mutant stalks them. Jones is fatally attacked and disemboweled, forcing Lutz to fight solo. Donning a gas mask, Lutz attempts to kill the beast with poison gas, but the creature seems unfazed by the fumes. A moment of opportunity comes when he hurls acid in its face, stunning the monster long enough for Lutz to seize an axe and finally kill it, ending the immediate threat.
The final image hints at a larger threat, as a new mutation bursts from the ground, leaving the town—perhaps the country—facing an ongoing nightmare that science and courage alone may not quell.
