In the fictional town of Pottsville, Idaho citizens begin disappearing. Young Michael Smith, son of Marge Smith (Dorothy Malone) is the first to vanish. A young man is decapitated while fleeing from an unseen assailant, and patrons in a drive-in theater are brutally murdered inside their vehicles. At each scene a pile of green slime is found. Wanting to get to the bottom of the disappearances but afraid that the publicity might damage the town's potato business, Mayor Gordon Lane (José Ferrer) hires Chemical engineer Garcon Jones (Martin Landau) to investigate. Also investigating the disappearances is Detective Mortimer Lutz (Bill Osco), who senses that something terrible has befallen the town.