In the Antarctic, a Norwegian helicopter pursues an Alaskan Malamute to an American research station. Upon landing, the passenger accidentally drops a thermite charge, which destroys the helicopter and kills the pilot when he tries to retrieve it. The passenger pursues the dog, firing a rifle, until he is shot dead by Garry, the station commander. The Americans send a helicopter pilot, MacReady, and Dr. Copper to the Norwegian camp for answers, but they find only a charred ruin. Inside, they find a corpse with its throat and wrists slashed in the radio room. Outside, they discover the burned remains of a humanoid corpse with two faces, which they bring back along with some video tapes. Their biologist, Blair, performs an autopsy on the corpse, finding a normal set of human internal organs.
Clark kennels the Malamute with the station's sled dogs; it soon metamorphoses and attacks them. When he hears the commotion, MacReady pulls the fire alarm, and Childs incinerates the creature. Blair performs another autopsy which leads him to believe the creature perfectly imitates other organisms. The Norwegians' records lead the Americans to a buried flying saucer that the station's geologist, Norris, hypothesizes is likely over 100,000 years old. Blair becomes increasingly paranoid and withdraws, calculating that if the alien escapes to a civilized area, all life on Earth will be assimilated within a few years. Fuchs tells MacReady that he is worried about Blair, and according to Blair's journal, the creature's "dead" remains are still active on a cellular level. The camp enacts safety measures designed to reduce risk of assimilation.
The creature assimilates Bennings, but Windows catches him outside before his metamorphosis is complete and MacReady burns the creature before it can escape. They discover Blair has wrecked all the transports and killed the remaining sled dogs. The team corners Blair as he is destroying the radio and locks him in an isolated tool shed. Copper recommends a blood-serum test to determine who is assimilated, but the paranoid men turn on each other when they find the blood stores have been sabotaged.
MacReady takes charge and orders Fuchs to continue Blair's work, but Fuchs disappears; MacReady, Windows, and Nauls find his burned body outside. Windows returns to warn the others while MacReady and Nauls investigate further. On the way back, Nauls cuts MacReady loose from the tow line. He assumes that MacReady has been assimilated when he finds a torn shirt with MacReady's name on it. As the team debates MacReady's fate, he breaks in and threatens to destroy the station with a bundle of dynamite if they attack him. Norris appears to suffer a heart attack after he and Nauls unsuccessfully attack MacReady from behind.
When Copper attempts to revive him, Norris transforms and kills Copper. MacReady incinerates the creature and orders Windows to tie up everyone for a new test. Clark attacks MacReady, and MacReady shoots him in the head, killing him. MacReady explains his theory that every piece of the alien is an individual organism with its own survival instinct. One by one, MacReady tests everyone's blood with a heated piece of copper wire, including Copper and Clark's bodies. Everyone is still human except Palmer, whose blood flees from the hot wire. Exposed, Palmer metamorphoses and attacks Windows, forcing MacReady to burn them both.
Leaving Childs on guard, the others head out to test Blair, only to find that he has tunneled out of the tool shed. They realize that Blair is assimilated and has been scavenging equipment to build a small escape craft. Discovering that Childs is missing and the station's power generator is destroyed, MacReady speculates that the Thing now intends to hibernate until a rescue team arrives. MacReady, Garry, and Nauls decide to dynamite the complex, hoping to destroy the Thing. While setting the explosives, Garry is killed and Nauls disappears. As MacReady rushes to set the charges, Blair, now metamorphosed into a larger monster emerges from below and destroys the detonator. MacReady triggers the blast with a stick of dynamite, destroying the abomination and the base.
MacReady sits nearby as the camp burns, and Childs reappears. Childs claims he was lost in the storm, pursuing Blair. They discuss their prospects of survival and distrust of each other, eventually deciding to stay put and "see what happens". MacReady offers Childs a bottle and chuckles.

Question: Why does Raul cut MacReady from the tow line?
Answer: Because he thinks he has been assimilated.
Alternative answer: Nauls cuts MacReady from the tow line because he thinks MacReady has bee assimilated