When the Syndicate suspect that one of their members is passing information to Mulder and Scully, they organize a canary trap to find the leak, using information about the safety of Mulder's mother as bait. X's role as an informant is discovered, and he is shot dead, although he is able to pass along the name of another informant who can be of use to Mulder—Marita Covarrubias, the Special Representative to the Secretary-General of the United Nations. Covarrubias' aid is sought when Mulder attempts to reach Tunguska in Russia to investigate the source of a further black oil contamination. Whilst there, Mulder is held in a gulag and used as a successful test subject for a black oil vaccine. He escapes and is able to return to America, having found that Alex Krycek is working with the Russians.

Having been diagnosed with cancer, Scully is unsure of her future with the FBI. Mulder is convinced that her condition is a result of her earlier abduction, and is prepared to make a deal with the Syndicate to find a cure. He is dissuaded by Walter Skinner, who secretly makes such a deal instead. While being pursued by an assassin responsible for a hoax alien corpse discovered on a mountaintop, Mulder fakes his own suicide, mutilating the assassin's face to provide a decoy body. He uses the distraction this offers to infiltrate The Pentagon to find a cure for Scully's cancer, while Scully is able to uncover and reveal a Syndicate connection within the FBI.