The Thing is a 1982 film directed by John Carpenter and written by Bill Lancaster, based on a novella from 1938 titled Who Goes There? If there is anything audience members remember about the film, it's the special effects that were done so well (for the time) that they repulsed and disgusted audiences. Although it was met with initial criticism and lack of interest, once The Thing was released to home video it began to find its audience and gain a cult following—forever immortalized as one of the greatest sci-fi horror films ever made.
Considering how long humans have been around, filmmaking is still a relatively new concept, but many believe The Thing was made during an era of pure filmmaking concerned with nothing but the art of the moving picture and having a fun escapist story. There weren't a lot of computer-generated films in 1982, as the first film to ever use CGI was only made 9 years prior to The Thing titled Westworld—a film about a futuristic western-themed park where the androids malfunction and kill the visitors. So, The Thing relied more on practical effects, which helped make it a classic.
It's always better to film something and enhance it with CGI than to film nothing at all. But those special effects helped to make whatever the monster is in The Thing a lot scarier. Something about the practical effects of the monster help to ground it in reality and make it that much more threatening. Whatever the monster is, it is seen trying to unsuccessfully impersonate different people and animals around it, suggesting it is a shapeshifting creature of some kind. The story starts off with a helicopter flown by a Norwegian pilot following a sled dog trying to kill it.
They are headed for an American research station in Antarctica. An accident causes the helicopter to blow up, and the Norwegian pilot tries to tell the Americans something, but they cannot understand him and he is then killed in self-defense after he tries to shoot at the Americans. In the charred rubble of the crash, there are also frozen and burned corpses, including the deformed human-like remains of a figure. An autopsy reveals the creature to have human organs, meaning this was most likely a human infected with the creature. The dog is put into a kennel, but something about it and the situation doesn't seem right.
The "dog" transforms, absorbs the other dogs, and is killed by a flamethrower. The crew discovers (after another autopsy) that this creature has the ability to shape-shift, imitate other beings, and take over another living thing. And, after the crew is able to recover some lost Norwegian data, they find the partially buried spacecraft of an alien which the crew estimates is at least 100,000 years old. There is also a dig site nearby.
After seeing some of the sled dogs, as well as one of the crew members, almost get absorbed (or assimilated) a paranoid crew member makes it to where no one can leave the station by killing the remaining sled dogs and dismantling the vehicles. They also make radio contact impossible. The rest of the crew fall victim to paranoia and distrust of one another, in constant fear of being assimilated. The 'thing' whatever it is, can look like anyone it wants. If a part of it falls off, that individual part can exist on its own, which means it can spread faster and infect more people.
Every time one of the crew becomes infected or assimilated, they become part of the Thing and must be incinerated to be destroyed. Characters, once they become part of the 'thing,' are called whatever their name was plus thing; for example Blair becomes Blair-Thing. After one of the crew members that has become part of the Thing transforms into a giant creature, the remaining crew members take him and the entire base out with dynamite.
Now without any way to escape or communicate and without any shelter, the two crew members (who are now freezing to death in the open air of Antarctica) decide not to spend their last moments being paranoid of one another and instead share a bottle of alcohol. So what was the monster thing anyway? It has the ability to shape-shift and even communicate, and it assimilates other life forms to survive. The Thing is essentially an extraterrestrial parasitic bacteria or organism whose only goal is to spread and multiply. It only knows survival, and survival means becoming the most dominant life form on Earth.
It is implied that the creature in The Thing has been in Antarctica for 100,000 years, but has been hibernating except for when given the opportunity to spread. The only way for the two crew members at the end of the movie to make sure this thing doesn't spread is to stay put and freeze to death (like the frozen corpses in the beginning of the film). This was John Carpenter's choice to end the movie this way, as in the novella it ends less hopefully for humanity with the crewmates still victorious but with the heavily implied foreshadowing that the birds they see flying above them have been infected.