Nobody trusts anybody any more.((Note: This cosplay account has, so far, been changed twice. So, if you're wondering wtf I'm doing on your friends list, it's because I've been changed.))
In winter 1982, an American Antarctic research station is alerted by gunfire and explosions. Pursued by a Norwegian helicopter, an Alaskan Malamute makes its way into the camp as the science station's crew looks on in confusion. Through reckless use of a thermal charge, the helicopter is destroyed and its pilot killed shortly after landing. The surviving passenger fires at the dog with a rifle, grazing Bennings (Peter Maloney), one of the American researchers. The passenger is subsequently shot and killed by Garry (Donald Moffat), the station commander. Not knowing what to make of the incident, the station crew adopts the dog.
Unable to contact the outside world via radio, helicopter pilot R.J. MacReady (Kurt Russell) and Dr. Copper (Richard Dysart) risk a flight to the Norwegian camp to find it destroyed, its personnel missing or dead. Finding evidence that the Norwegians had dug something out of the ice, the pair return to the station with the partially-burned remains of a hideous creature which bears some human features. An autopsy of the cadaver by Dr. Blair (Wilford Brimley) is inconclusive, save to find that the creature had what appeared to be a normal set of internal organs.
At Bennings' request, the station's wrangler, Clark (Richard Masur), kennels the stray with the rest of the station's sled dogs. Noises from the kennel cause Clark to return, finding almost the entire sled team in the process of being messily assimilated by the stray dog, which has transformed into a monster. MacReady summons the rest of the crew to the kennel with the fire alarm and orders Childs (Keith David) to incinerate the creature with a flamethrower. A subsequent autopsy by Blair reveals that the stray dog was an alien capable of absorbing and perfectly imitating other life-forms. Realizing the implications of this, Blair quickly becomes withdrawn and suspicious of the rest of the crew. A second helicopter expedition discovers an alien spacecraft unearthed by the Norwegian research team, revealing that the creature had awakened after being buried within the ice for many thousands of years.
Bennings and Windows (Thomas G. Waites) quarantine the burnt remains of both the dog-creature and the Norwegian cadaver in the storage room, but in the process Bennings is left alone. Moments later, Windows discovers Bennings in the process of being replicated. The crew burns the Bennings replica before its transformation is complete. Determining that all life on Earth would be assimilated in just over three years if the creature were to reach another continent, Blair goes berserk, destroying the helicopter and radio equipment and killing the remaining sled dogs, thus containing further contamination. The team overpowers him and confines him in the tool shed. With all contact to the outside world cut off, the crew wonders how to determine who is still human. Paranoia quickly sets in as the first attempt to develop a test using uncontaminated blood samples is sabotaged by an unknown party.
Fuchs (Joel Polis), attempting to continue Blair's research, goes missing shortly afterwards during a power failure. While searching for Fuchs' body, MacReady comes under suspicion and is locked outside in a severe blizzard. Somehow finding his way back to camp without a guide line, MacReady breaks into a storage room and threatens the rest of the crew with dynamite. In the course of the standoff, Norris (Charles Hallahan) suffers a heart attack. When Dr. Copper attempts to revive him by defibrillation, Norris' body transforms and kills Copper. Norris' head detaches from his body, sprouts legs and attempts to escape as the others burn the body, leading MacReady to theorize that every piece of the alien is an individual animal with its own survival instinct. In an altercation that precedes a test proposed by MacReady, Clark tries to stab MacReady with a scalpel, who shoots and kills him in self-defense. The rest of the crew complies with the test; blood samples are drawn from each member of the team and jabbed with a hot wire to see whose blood will react defensively. Palmer (David Clennon), the backup pilot, is soon unmasked as an imitation, and manages to kill Windows before being set alight and blown up with dynamite by MacReady. He then torches Windows' body with a flamethrower as it begins to transform.
Confirming that MacReady, Childs, Garry, and Nauls (T.K. Carter) are still human, the surviving crew set out to administer the test to Blair, only to find that he has escaped. After they discover that Blair had been constructing a small flying craft of alien design underneath the tool shed and witness Childs inexplicably abandoning his post at the main gate, the facility loses power. Realizing that the creature now wants to freeze again so a future rescue team will find it, the remaining crew acknowledge that they will not survive and set about destroying the facility with dynamite and Molotov cocktails in hopes of killing the creature. While setting explosives in the underground generator room, Garry is killed by the infected Blair. Nauls follows the sounds of the creature and is never seen again. Alone, MacReady prepares to detonate the charges when the creature, larger then ever, emerges from beneath the floor. MacReady kills it with a stick of dynamite, which sets off the rest of the charges and destroys the entire facility.
After some time, MacReady is shown wandering alone in the flaming rubble. He encounters Childs, who claims to have seen Blair and gotten lost while chasing him in the snow. With the polar climate closing in around them, they acknowledge the futility of their distrust, sharing a drink as the camp burns.