The X-Files, S1E8 - "Ice"

Mulder and Scully are called upon to travel to Icy Cape, Alaska to investigate what happened to the scientists of the Arctic Ice Core Project, who have gone silent after sending a disturbing transmission. Mulder and Scully travel to Icy Cape along with a physician, Dr. Hodge; a toxicologist, Dr. Silva; a geophysicist, Dr. Murphy; and a pilot named Bear. Upon arriving at the base, they discover the project team all dead; two are determined to have committed suicide. While investigating, Mulder is attacked by a dog, which bites Bear. The agents and doctors discover that the dog has black nodules on its skin, as well as something moving visibly inside its neck. Bear examines himself and discovers he has developed the same nodules. The doctors discover a living micro-organism in both the blood of the dead scientists and their ice samples. Bear and Hodge desire to leave the base immediately, while Mulder counters that they could be infected with this unknown organism and run the risk of spreading it if they return to civilization. He suggests that they all be tested for infection. Bear resists, attacking Mulder; the others realize that Bear is infected, and Hodge removes a wormlike organism from his neck. Bear then dies. Mulder attempts to call for help, but is informed that a storm will make rescue impossible for several days.

The agents and doctors speculate on the nature of the parasite, with Hodge postulating that it survives on acetylcholine, causing those infected to act violently. Mulder believes that the origin of the parasite is extraterrestrial, considering that it was extracted from ammonia-rich ice in a meteor crater. Mulder and Scully argue about whether to destroy the parasite, while Hodge and Silva begin to distrust Mulder and Scully. To remove suspicion, the survivors inspect each other for the black nodules before going to bed.

In the night, Mulder wakes up to the sound of a door closing. He investigates and discovers Murphy's dead body shoved in a freezer. The other survivors arrive, and Hodge accuses Mulder of killing Murphy; Mulder declares that one of the others is infected and refuses to allow Hodge to give him a blood test, claiming that Hodge could be infected and cannot be trusted. Mulder, Hodge, and Scully have a standoff that results in Mulder being locked in a storeroom. Hodge, Silva, and Scully continue to examine the parasite and discover that two parasites introduced to the same host will kill each other. They infect the dog with one of the two surviving worms, and it appears cured. Scully and Mulder inspect each other, discovering that neither is infected. Scully declares that either Hodge or Silva must be infected. Hodge and Silva attack Mulder and attempt to infect him with the worm, but at the last minute, Hodge sees a worm moving within Silva's neck, realizing that she is infected. Silva runs from the others and attempts to fight, but Mulder and Scully overpower her and Hodge administers the worm to her, curing her.

The survivors are rescued by air, with Silva being taken to quarantine and Mulder, Scully, and Hodge being released. Mulder expresses a desire to return to the base to investigate the parasite further in case it is ever released somehow. Hodge informs him that the base was destroyed after their rescue.

"Ice" is perhaps the first truly great episode of The X-Files. The script is essentially a retelling of John Carpenter's The Thing; we have a group of people in an isolated arctic setting, the presence of an entity that corrupts and takes over those infected with it, and the resulting breakdown of trust among the survivors. There's also an element of Lovecraft present in the idea of an ancient extraterrestrial organism that predates man lying dormant for millennia, only to be reawakened with disastrous consequences. The themes of isolation and distrust raise the stakes to life-or-death levels, with Mulder and Scully pitted against each other for the first time, escalating the suspense and tension to sublime levels.