S1E8 · Ice

A mass murder–suicide occurs among a team of geophysicists at an outpost in the Icy Cape of Alaska. FBI agents Fox Mulder and Dana Scully head for the outpost, accompanied by physician Dr. Hodge, toxicologist Dr. Da Silva, geologist Dr. Murphy, and the team's pilot Bear. Along with the bodies of the geophysicists, the group finds a dog that attacks Mulder and Bear. Scully notices black nodules on its skin and suspects that it may be infected with bubonic plague, though she also notices a rash on its neck and movement beneath its skin. Having been bitten by the dog, Bear becomes ill and develops similar nodules on his body, but autopsies reveal no such nodules on the bodies of the geophysicists.

Murphy finds an ice core sample believed to have originated from a meteor crater and theorizes that the sample might be 250,000 years old. Although Bear insists on leaving, the others are concerned about infecting the outside world. When he is asked to provide a stool sample, Bear attacks Mulder and tries to flee. Something moves under his skin, and he dies when Hodge makes an incision at the location of the movement and removes a small worm from the back of his neck. Now without a pilot, the group is informed that evacuation is impossible for a while because of an oncoming storm.

The worm removed from Bear is kept in a jar, while another is recovered from the body of one of the geophysicists. Believing that the worms are extraterrestrial, Mulder wants them kept alive, but Scully feels they should be destroyed to prevent more infections. The group check each other for black nodules and find none, though Mulder reminds Scully that the nodules disappeared from the dog over time. He wakes up in the night to find Murphy in the freezer with his throat slashed; when the others arrive to see him standing over it, all of them—including Scully—lock Mulder in a storage closet as they suspect he has become infected and killed Murphy.

Scully discovers that two worms placed in the same host will kill each other. When the group investigates by putting a second worm into the infected dog, leading the dog to recover when the worms destroy each other. Against Scully's objections and after trapping her in a freezer, Hodge and Da Silva try to put the other worm into Mulder, but Hodge sees movement under Da Silva's skin and realizes she is the infected one who killed Murphy. Da Silva breaks free and the rest pursue her through the outpost until Mulder and Scully restrain her, allowing Hodge to place the last worm inside her.

After they are evacuated, Da Silva and the dog are quarantined and the others are released after showing no signs of infection. When Mulder later declares that he wants to return to the site, Hodge tells him that it has been destroyed by the government.