When communication is cut off with a research facility in Alaska - and the last words of anyone stationed there are, "We are not who we are" - Mulder and Scully, accompanied by a team of physicians and scientists, proceed to Alaska, where they discover a parasitic wormlike creature that takes control of the human mind and intensifies feelings of fear and paranoia, with deadly results.

When they arrive, wanting to find out what happened, the first thing they deduce is that for some reason, the trust had completely died within the group. Soon after, Bear, the pilot is attacked and bitten by a dog who seems to have gone crazy. When the dog is examined, Scully finds some sort of parasite inside it. They are beginning to realize what is going on here especially when the pilot starts going crazy. It turns out he has also been infected by the parasite.

After numerous experiments, Scully and some of the other scientists come to the conclusion that the parasitic worm is attaching itself to the hypothalamus gland and causing the host to go crazy and lose control of their bodies. But when Dr.Denny Murphy is found murdered and Mulder is right under him, Scully and the others must decide whether Mulder has been infected. Their choice is made easy when Mulder becomes agitated and angry. While he is locked up, Scully finds that two parasites in the same host will kill each other off essentially destroying the virus. But shall they inject Mulder with one of them? One parasite in a body will infect someone but has he already got one inside him?

It turns out to be one of the scientists who has been infected. M & S can now escape the dreadful place but when Mulder asks to go back and examine the site for more information on the parasite it is revealed that the government have torched the place and destroyed any trace of the worm. It just shows how a friendship can be tested so easily.