Mulder and Scully view a videotape recorded by two geophysicists who were part of the Antarctic Ice Core Project, a scientific expedition sanctioned by the United States Government. Their mission was to drill deep into the Arctic ice and collect core samples that date the Earth's climate as far back as the Dawn of Man. Something went wrong during the expedition, leaving the team dead and the two geophysicists killed by their own hand.

Mulder and Scully are assigned to investigate the case. They are teamed with Dr. Denny Murphy, a geologist; Doctor Lawrence Hodge, a physician, and Doctor Nancy Da Silva, a toxicologist. The group boards a plane piloted by Bear, a rugged man who wears a bear claw around his neck.

When the group arrives at the Arctic compound, they busily set about documenting their grisly surroundings. Scattered around the site are the bodies of scientists from the previous expedition, all of whom died mysterious, violent deaths.

A dog suddenly springs from the darkness and jumps onto Mulder. Bear intercedes, and in the process of pulling the animal off Mulder, he is bitten. The dog is sedated, and during an examination, a worm-like organism is discovered squirming beneath its skin.

Scully's autopsies reveal that the scientists from the previous expedition-except for the two who committed suicide-killed each other. Examination of ice core samples leads to the conclusion that the first team tapped into the remnants of a meteor that collided with the Earth thousands of years in the past. When the scientists came in contact with the core samples, they exposed themselves to an alien organism that had remained dormant beneath the Arctic Ice.

When Bear is told his blood and stool samples must be analyzed to ensure the parasite has not infected him, he refuses to cooperate. A scuffle ensues, and as the others pin Bear down, Hodge discovers-and surgically removes-one of the parasites from his body. When the worm is removed, however, Bear dies.

The scientists theorize that the parasite feeds on a chemical that controls aggressive behavior, leading the first team of geophysicists to kill one another. Paranoia within the group builds to a fever pitch when a severe storm ends radio contact with the rest of the world.

Mulder discovers Murphy's body stuffed inside a refrigerator, his throat cut. The others suspect that Mulder may be the killer and padlock him inside a supply room.

While performing an experiment, Hodge accidentally mixes blood samples from two different infected hosts. To his surprise, the parasitic organisms attack and kill one another. The scientists infect the dog that attacked Bear with a second worm. It returns to normal.

Mulder convinces Scully that he is not infected. When they exit the supply room, the pair are jumped by Da Silva and Hodge, who pin Mulder to the ground and lower the last worm to his ear. But Hodge is horrified when he discovers the parasite wriggling beneath Da Silva's skin. The others hold Da Silva down while Hodge inserts the parasite.

The group is rescued and flown to another base. Mulder announces he is returning to the site to perform more tests. Hodge informs him that the military burned the compound to the ground.