In Antarctica, a Norwegian helicopter pursues a sled dog to an American research station. The Americans witness the Norwegian passenger accidentally blow up the helicopter and himself. The Norwegian pilot fires a rifle and shouts at the Americans, but they cannot understand him and he is shot dead in self-defense by station commander Garry. The American helicopter pilot, R.J. MacReady, and Dr. Copper leave to investigate the Norwegian base. Among the charred ruins and frozen corpses, they find the burned remains of a malformed humanoid which they recover to the American station. Their biologist, Blair, performs autopsies on the remains and finds a normal set of human organs.

Clark kennels the sled dog, and it soon metamorphoses and absorbs the station dogs. This disturbance alerts the team and Childs uses a flamethrower to incinerate the creature. Blair autopsies the new creature and learns that it can perfectly imitate other organisms. Recovered Norwegian data leads the Americans to a large excavation site containing a partially buried alien spacecraft, and a smaller, human-sized dig site. Norris estimates that the alien ship has been buried for at least 100,000 years. Blair grows paranoid that the creature could assimilate all life on Earth in a matter of years. The station implements controls to reduce the risk of assimilation.

The "dead," malformed humanoid creature assimilates an isolated Bennings, but Windows interrupts the process and MacReady burns the Bennings-Thing. Blair sabotages all the vehicles, kills the remaining sled dogs, and destroys the radio to prevent escape. The team imprisons him in a tool shed. Copper suggests a test to compare each member's blood against uncontaminated blood held in storage, but after learning that the blood stores have been destroyed, the men lose faith in Garry, and MacReady takes command.

MacReady, Windows and Nauls find Fuchs's burnt corpse and surmise he committed suicide to avoid assimilation. Windows returns to base while MacReady and Nauls investigate MacReady's shack. On their return, Nauls abandons MacReady in a snowstorm, believing he has been assimilated after finding his torn clothes in the shack. The team debate whether to allow MacReady inside, but he breaks in and holds the group at bay with dynamite. During the encounter, Norris appears to suffer a heart attack.