A United States Air Force crew is dispatched from Anchorage, Alaska at the request of Dr. Carrington (Robert Cornthwaite), the chief scientist of a North Pole scientific outpost. They have evidence that an unknown flying craft has crashed in their vicinity, so reporter Ned Scott (Douglas Spencer) tags along for the story.
Dr. Carrington later briefs Captain Hendry (Kenneth Tobey) and his airmen, and Dr. Redding (George Fenneman) shows photos of a flying object moving erratically before crashing—not the movements of a meteorite. Following erratic magnetic pole anomalies, the crew and scientists fly to the crash site where the mysterious craft lies buried beneath refrozen ice. As they spread out to outline the craft's general shape, the men realize they are standing in a circle; they have discovered a crashed flying saucer. They try de-icing the buried craft with thermite heat bombs, but only ignite its metal alloy, causing an explosion that destroys the saucer. Their Geiger counter then points to a slightly radioactive frozen shape buried nearby in the refrozen ice.
They excavate a large block of ice around what appears to be a tall body and fly it to the research outpost, just as a major storm moves in, cutting off their communications with Anchorage. Some of the scientists want to thaw out the body, but Captain Hendry insists on waiting until he receives further instructions from the Air Force. Later, Corporal Barnes (William Self) takes the second watch over the ice block and to avoid looking at the body within, covers it with an electric blanket that the previous guard left turned on. As the ice slowly melts, the Thing inside revives; Barnes panics and shoots at it with his sidearm, but the alien escapes into the raging storm. The Thing is attacked by sled dogs and the airmen recover a severed arm.
A microscopic examination of a tissue sample reveals that the arm is vegetable rather than animal matter, demonstrating that the alien is a very advanced form of plant life. As the arm warms to ambient temperature, it ingests some of the dogs' blood covering it, and the hand begins moving. Seed pods are discovered in the palm. The Air Force personnel believe the creature is a danger to all of them, but Dr. Carrington is convinced that it can be reasoned with and has much to teach them. Carrington deduces their visitor requires blood to survive and reproduce. He later discovers the body of a dead sled dog hidden in the outpost's greenhouse. Carrington has Dr. Voorhees (Paul Frees), Dr. Olsen (William Neff) and Dr. Auerbach stand guard overnight, waiting for The Thing to return.
Carrington secretly uses blood plasma from the infirmary to incubate seedlings grown from the alien seed pods. The strung-up bodies of Olsen and Auerbach are discovered in the greenhouse, drained of blood. Dr. Stern is almost killed by the Thing but escapes. Hendry rushes to the greenhouse after hearing about the bodies, and is attacked by the alien. Hendry slams the door on the Thing's regenerated arm as it tries to grab him. The alien then escapes through the greenhouse's exterior door, breaking into another building in the compound. Nikki Nicholson (Margaret Sheridan), Carrington's secretary, reluctantly updates Hendry when he asks about missing plasma and confronts Carrington in his lab, where he discovers the alien seeds have grown at an alarming rate. Following Nicholson's suggestion, Hendry and his men lay a trap in a nearby room: after dousing the alien with buckets of kerosene, they set it ablaze with a flare gun, forcing it to jump through a closed window into the Arctic storm.
Nicholson notices that the temperature inside the station is falling; a heating fuel line has been sabotaged by the alien. The cold forces everyone to make a final stand near the generator room. They rig an electrical "fly trap", hoping to electrocute their visitor. As the Thing advances, Carrington shuts off the power and tries to reason with it, but is knocked aside. On Hendry's direct order that nothing of the Thing remain, it is reduced by arcs of electricity to a smoldering pile of ash; Dr. Carrington's growing seed pods and the Thing's severed arm are destroyed as well.
When the weather clears, Scotty files his "story of a lifetime" by radio to a roomful of reporters in Anchorage. Scotty begins his broadcast with a warning: "Tell the world. Tell this to everybody, wherever they are. Watch the skies everywhere. Keep looking. Keep watching the skies."