In outer space, a flying saucer careens into Earth's atmosphere. In Antarctica, winter 1982, a man riding in a helicopter fires a rifle several times at a dog on the snow-covered terrain below but the fleeing animal escapes harm. At the nearby United States National Science Institute Station 4, team members, among them R. J. "Mac" MacReady, relax in the recreation room, but are interrupted when the hunt works its way onto Institute grounds. Hearing the helicopter, the team gathers outside where the gunman has since exchanged bullets for grenades. As the dog dodges the explosives, the gunman fails to recover a live grenade he dropped in the snow and the resulting explosion kills the pilot and destroys the copter. The gunman, shouting furiously in Norwegian, continues on foot and the dog seeks refuge amidst the team. The gunman shoots at the dog, but hits one of the Institute scientists, George Bennings, in the leg. The gunman then pursues the dog deeper into the compound, firing more shots, until Garry, the Institute's commander, pulls a handgun and shoots the gunman dead. With safety restored, the group extinguishes the blazing helicopter, which MacReady later discovers was loaded with cans of kerosene. The on-site physician, Dr. Copper, sutures Bennings's flesh wound and speculates that the Norwegian’s behavior was the result of cabin fever. The radio operator, Windows, attempts unsuccessfully to report the incident to authorities off the continent. After identifying the Norwegians as members of a ten-party science expedition, Doc volunteers to fly to their base to check on the remaining expedition members and MacReady is selected to pilot his helicopter. When Mac and Doc arrive at the Norwegian station, they find it in disarray, exposed to the elements and frozen throughout. Inside they discover a bloody ax and the mutilated and frozen corpse of one of the expedition members. They also uncover a large hollowed-out block of ice that Doc guesses might have contained an excised fossil. Behind the station, they find smoldering, unidentifiable remains, though it appears to contain some features of a man. Beside it, they find cans of kerosene. Mac and Doc return to the Institute bringing with them the frozen corpse, the smoking remains, and some of the Norwegians' research. Disturbed and baffled by the specimens, Doc and Blair begin autopsies, which reveal a normal set of internal organs inside the remains. Team member Clark locks up the new dog with the Institute's dog pack, but after he leaves, the dog moans and shakes; then its face splits open as tentacles sprout from its body. A dog in the pack panics and attempts to gnaw its way through the chain link fence when the creature sprays it with slime. Upon hearing the panic, Clark returns and when he opens the pen door, two dogs bolt to safety before Clark can re-latch the door on the creature, which is still whipping itself violently about the pen. Mac, hearing the commotion, signals the team and calls for Childs, another team member, to bring the flamethrower. From outside the pen, the team watches as the creature lashes out its tentacles at the remaining dogs. Seeing this, the team fires several shots, wounding the creature and killing the remaining dogs. Moments later Childs arrives with the flamethrower and torches the creature, which has begun to sprout large arms and hands. The following morning Doc performs an autopsy on the remains, concluding that it is a shape-shifting organism that replicates the cellular makeup of the subjects it absorbs. Later, by questioning Clark, Blair learns that the creature, while disguised as a dog, wandered freely around the camp and that Clark was alone with it for over an hour. Though Clark senses Blair is suspicious of him, Blair denies it, saying, “It’s probably nothing.” Using the Norwegians' video materials, the team pinpoints an important excavation site that Mac is intent on investigating. At the site, Mac and a small team discover an excavated flying saucer, estimated to have crashed 100,000 years ago, as well as the hole in the ice where the Norwegians excised the creature. After Mac reports the trip's findings, Blair privately runs a computer simulation of the creature’s cellular absorption process. In addition to plotting how the cells of “the thing” attack, absorb, and mimic foreign cells, the simulation assesses a seventy-five percent probability that one or more team members may be infected. The simulation projects it would take 27,000 hours for the entire world to become infected if an intruder organism reaches civilized areas. At Doc's request, Windows and Bennings haul the two sets of burned remains down to the storeroom for safe keeping, after which Windows sets out to recover the keys to lock the room. Neither Windows nor Bennings, who stays behind, notices the subtle movements of one set of remains. Meanwhile, Blair locks himself away in his room and refuses to answer the door, and the team learns, through Blair’s notes, that there is still cellular activity in the creature’s burned remains. When Windows returns to the storeroom, he finds it alive and in the process of absorbing Bennings. Windows runs for help and when he returns to the storeroom with Mac and Fuchs, Bennings has broken through an exterior window and is stumbling in the snow. They catch up with Bennings and are followed by others who respond to the commotion, but realize it is not Bennings they pursued. It is the creature, which looks much like Bennings but has mutated hands. The creature lets out a resonating wail, before Mac douses it in gasoline and sets it ablaze. After the team cremates the remains in a makeshift fire pit, Fuchs reports that Blair is missing. Mac spots Blair running from the Institute's helicopter with an ax in hand and discovers he destroyed the helicopter's controls. Retreating inside, the armed Blair threatens to shoot anyone who tries to stop his destruction of the Institute's communication equipment. Childs tells Mac that Blair also disabled the tractor and killed the remaining dogs. While ranting about “that thing” needing to be human to make it to the mainlands, Blair shoots at anyone attempting to stop his rampage, but as soon as he empties his gun, the team rushes and disables him. They lock Blair in the tool shed, where he warns Mac to "watch Clark and watch him close." The team, hit with the realization that somebody in the camp is not what he seems, decides to test each other's blood to determine who is infected. However, they find that the supply of clean blood they need to perform the test has been emptied and that the lock on the refrigerator where the blood is stored is unbroken, indicating the saboteur used a key. This discovery makes Doc and Garry the prime suspects, as they are the only two with access to the refrigerator. Accusations and an armed standoff ensue. Eventually, Garry relinquishes command of the team to Mac, despite his claim of innocence, because he realizes his reliability is in question. Mac orders that Garry, Doc, and Clark be sedated, while Fuchs works on a new test. In the meantime, Mac notes in a secret audio log his theory that the creature tears through the subject's clothes during the absorption process. Moments after Mac leaves a brief meeting with Fuchs, someone kills the electricity in the lab where Fuchs is working. Fuchs stumbles through the dark, relying on the weak light of a candle, when a figure passes before him. He calls out, but no one answers. Outside the lab, Fuchs finds a shredded jacket with the name "R. J. MacReady" stenciled on the back. Some time later, in the recreation room, Mac questions the group about the blackout and organizes search teams to look for Fuchs, who is now missing. After checking on Blair, MacReady, Nauls, and Windows discover what appears to be Fuchs's burned remains outside, in the snow. Mac also observes that the lights in his shack are on, and he sends Windows to report the news to the team, while he and Nauls continue on to the shack. Mac and Nauls have been missing for about forty-five minutes, when Childs gives the order to seal the outside doors. However, Nauls returns before all the doors are sealed. He informs the team that he cut Mac loose from the guide line, stranding him in the ongoing blizzard, after he uncovered Mac's shredded jacket. Most of the team concede that Mac is now "one of those things," and, while Childs is not fully convinced, he refuses to open the door for whatever it is that starts rattling its handle. Soon after the rattling stops something breaks through the supply room window down the hall. Childs hacks through the locked supply room door with an ax to find Mac inside, holding a flare and dynamite and threatening to blow up the camp if anyone “messes” with him. He forces most of the team back into the hall but overlooks Nauls and Norris, who rush him from behind. In the scuffle Norris is knocked to the ground and later found not breathing. In an attempt to revive him, Doc places the pads of a defibrillator on Norris’s chest, which splits open sending Doc lurching into it. The cavity then quickly clamps shut, severing Doc's forearms and two “things” are created. One projects from out of Norris's chest and latches onto an overhanging vent. A second grows from within Norris's head, sprouts legs and scuttles toward the door. Mac eventually torches both creatures. He then gathers the remaining team members at gunpoint and demands they be tied up. During initial protests, Clark tries to stab Mac, but Mac shoots him first. With the team restrained, Mac begins a new test, based on the theory that each piece of an infected subject is occupied by a separate “thing” that will try to survive when threatened. Thus, the blood of an infected subject will try to flee when touched by a heated wire. The test clears Windows and Mac, as well as the deceased Doc and Clark, before exposing Palmer as infected by the creature. When his blood leaps from the petri dish, Palmer transforms and kills Windows. Mac fumbles with his malfunctioning flamethrower, finally igniting Palmer, then blowing him up with a stick of dynamite. He then turns the flames on Windows's corpse, which is beginning to transform. Mac's test also clears Nauls, Childs, and Garry, but when they go to the tool shed to test Blair, they find him missing, despite the door being locked from the outside. Although they do not find Blair, they discover he dug a tunnel below the tool shed in which he is building a flying saucer, using parts from the helicopter and other Institute technology. After the men resurface, Nauls sees Childs leaving the main building in which he was posting watch. When the Institute's electricity goes out, Mac concludes that the creature blew the generator, initiating its plan to freeze the camp and itself until a rescue team arrives to transport it to the main lands. Intent on stopping it, the men blow up its ship and set out to explode the camp. Blair kills Garry, as he is setting the final explosives, and Childs also goes missing, leaving Mac alone in a final battle with the “thing.” Using his last stick of dynamite, Mac blows it up and initiates a chain reaction that destroys the rest of the camp. As Mac sits down to drink a bottle of booze, Childs arrives, claiming he got lost in the storm while pursuing Blair. Both men sit and drink, realizing that if one of them is the “thing,” the other can do nothing about it.