In the midst of the Antartican snowfield, the scientists and workers of a small American research base are shocked when a helicopter begins to circle their camp, chasing and shooting at a dog. When the helicopter is destroyed and the passengers are killed, the dog is let into the base and the Americans begin to wonder what has actually happened. The helicopter has Norwegian markings, must be from the Norwegian base not too far from their own. A team of Americans are sent to the Norwegian base and find out what has happened. On arrival, they find that the place has been totally destroyed. They also discover a mangled body that looks as though it was once that of a person, which they bring back with them for further study. It is only then that the clues begin to add up; the dog morphs horribly into a strange creature that attacks the researchers. They manage to fight it off, but they come to a terrible conclusion: an alien with the power to transform and take the appearance of anybody else is amongst them. Who is infected already, and who can be trusted? Helicopter pilot R.J. MacReady sets out to find the answers to exactly that.