Turns out that the new "dog" is an alien in disguise. This alien can take the form of any living mammal it is exposed to and infects, with a brief incubation period where it resembles a mass of raw flesh, writhing snakes, and surrealistic horse heads.
Naturally, the Americans become immediately suspicious of each other, since any of them could be an alien waiting to emerge. Thus ensues a series of testosterone-laced bickering, fighting, and hunting, interrupted by episodes of crew members transforming into hideous aliens that can only be killed via flamethrowers.
How others will see it. It comes as little surprise that men like the movie more than do women, and that the young like it more than do the old. Nevertheless, the imdb.com user ratings are consistently very high with the exception of women over 45, who apparently are less excited at the prospect of watching men turn on one another, before turning one by one into hamburger and sausage links at regular intervals.
Although John Carpenter's horrorshow is highly popular today, it wasn't always the case. The box office was middling, critical reception was mixed at best, and The Thing was ignored by the Golden Globes and Academy Awards.
How I felt about it. A loose remake of the 1951 Howard Hawks film The Thing from Another World, but really, this is Alien (1979) with the setting changed from remote outer space to remote Antartica. You have a crew at odds with each other, who die one by one, with all of humanity threatened should the crew fail to stop the unkillable hideous creature.
Of course, there are differences as well, some of them surprising. There are no women, not even token hotties. There are two blacks instead of the expected one. Today, there would be an Hispanic, but such casting was unessential back in 1982.
For most viewers, the draw is the special effects, e.g. from men to spaghetti in thirty seconds. Intellectuals bored with Fermat's last theorem can instead amuse themselves with proving which crew members became infected when, and going to extreme lengths to cover any and all plot holes.
But along with such time-wasting diversions, John Carpenter does provide some insight into the human condition. In a crisis, all pretenses of friendship are cast aside, and the crew is united only in their desire to charbroil the alien(s) amongst us. As in Alien, one crew member, in this case Kurt Russell, rises to the occasion to provide leadership. He is opposed, but perseveres into the final scene.
Whether or not the alien succeeds is unimportant. After all, it is only a movie. There's no need to worry whether Fido will become the CGI version of Old Yeller. But it's comforting to know that someone with our best interests at heart will take charge: a Lincoln or FDR, instead of a Hitler or Mussolini.