In 1982, an alien spacecraft is discovered beneath the Antarctic ice by a Norwegian research team: Edvard (Trond Espen Seim), Jonas (Kristofer Hivju), Olav (Jan Gunnar Røise), Karl (Carsten Bjørnlund), Juliette (Kim Bubbs), Lars (Jørgen Langhelle), Henrik (Jo Adrian Haavind), Colin (Jonathan Lloyd Walker), and Peder (Stig Henrik Hoff). Paleontologist Kate Lloyd (Mary Elizabeth Winstead) is recruited by Dr. Sander Halvorson (Ulrich Thomsen) and his assistant Adam Finch (Eric Christian Olsen) to investigate the discovery. They travel to the Norwegian base in a helicopter manned by Carter (Joel Edgerton), Derek (Adewale Akinnuoye-Agbaje), and Griggs (Paul Braunstein). After viewing the spacecraft, Kate, Sander and Adam are told the group also discovered an alien body from the crash buried in the ice.
The body is brought to the base in a block of ice. That evening, while the team celebrates their find, Derek sees the alien burst from the ice and escape the building. The team searches for the creature and discovers that it killed Lars' dog. Olav and Henrik find the alien which then grabs and engulfs Henrik. The rest of the group arrive and set fire to the creature, killing both it and Henrik. An autopsy of the scorched alien corpse reveals that its cells are still alive and are consuming and imitating Henrik's own.
Derek, Carter, Griggs and Olav take the helicopter to seek help. Kate discovers bloody dental fillings near a blood-soaked shower. She runs outside to flag down the helicopter after it takes off. When it attempts to land, Griggs transforms into the Thing and attacks Olav, causing the helicopter to crash in the mountains. When Kate returns to the shower, she finds the blood is gone. The team agrees to evacuate, but Kate confronts them with her theory that the Thing can imitate them and has likely already done so. They dismiss her claims, but Juliette says she saw Colin leaving the showers. Juliette and Kate look for the vehicle keys to prevent the others from leaving, when suddenly Juliette transforms and attacks Kate. As Kate flees, she runs past Karl who is consumed by the creature instead. Lars arrives with a flamethrower and burns the Juliette-Thing.
Carter and Derek return to the base, but the team refuses to believe that they could have survived the crash. Kate has Carter and Derek isolated until a test can be prepared to verify they are human. Adam and Sander work on a test, but the lab is sabotaged. Kate proposes another test; believing that the Thing cannot imitate inorganic material, she inspects everyone and singles out those without metal fillings: Sander, Edvard, Adam, and Colin. Lars and Jonas go to retrieve Carter and Derek for testing, and discover they have broken out of isolation. As Lars searches near a building, he is suddenly pulled inside. The group hears Carter and Derek breaking into the building and rushes to intercept them. Edvard orders Peder to burn them.
Peder takes aim, but Derek now has a gun and shoots several times, killing Peder and rupturing the flamethrower's fuel tank which ignites. The explosion knocks Edvard unconscious. When brought to the rec room, Edvard transforms, infecting Jonas and Jameson before assimilating Adam. Kate torches the infected Jonas and Derek before she and Carter pursue the Thing. While the pair searches, Sander is also infected. After they separate, the Thing into which Edvard and Adam are fused corners Carter in the kitchen, but Kate burns it before it can attack. Kate and Carter see Sander drive off into the blizzard and pursue him in the remaining snowcat.
They arrive at the now-active spaceship. Kate falls into the ship and is separated from Carter. Confronted by the creature (which briefly uses Sander's face), Kate destroys it with an explosive grenade and the damage deactivates the ship. As Kate and Carter return to their vehicle, Kate accuses Carter of being a Thing because he is missing his earring. When she confronts him, Carter points to the wrong ear. Kate burns him and retreats to a snowcat. Whether she survives or not is left as a mystery, but Carter did mention that there is a Soviet base a few miles away, before Kate killed his Thing replicant.
The following morning, helicopter pilot Matias arrives at the base. Colin is shown to have committed suicide in the radio room. Matias sees the burned remains of the Adam/Edvard alien in the snow. Lars orders Matias at gunpoint to show his dental fillings to prove he is human. The Thing, in the form of Lars' deceased dog, runs out of the camp, and Lars and Matias give chase in the helicopter, thus setting the events of John Carpenter's The Thing in motion.