S4E9 · Terma

While imprisoned in a gulag in Krasnoyarsk, Fox Mulder (David Duchovny) learns that Alex Krycek (Nicholas Lea) is a double agent working for the Russian taskmasters and that all prisoners have been subjected to a black oil vaccine. In his dungeon, Mulder is given a sharp object by another prisoner, who had made it to commit suicide. When all the prisoners are taken out and are arranged in a line, Mulder sees Krycek talking to one of the captors and runs toward them, tackling them. Mulder then steals a truck and flees with an unconscious Krycek in the back of it; a pursuit by his captors ensues.

As Mulder drives the truck, Krycek awakes and jumps off the truck. Mulder, having been unable to stop the truck due to its faulty brakes, runs the truck off the road and it crashes. Mulder quickly makes his way out of the truck and hides under the dry leaves, while Krycek is found by a group of men whose left arms have all been amputated and has his arm forcibly severed to prevent his involvement in black oil vaccination tests.

Meanwhile, Vasily Peskow (Jan Rubeš), a former KGB agent, has come out of retirement and traveled to the United States where he assassinates Bonita Charne-Sayre, a doctor who is developing a black oil vaccine for the Syndicate. He then tracks down Charne-Sayre's test subjects and tests the Russian vaccine on them before killing them to cover his tracks.

Mulder is found by a group of Russian peasants (one of whom owns the truck that Mulder stole), who help him return to the United States, and rejoins his partner Dana Scully (Gillian Anderson). Scully and assistant director Walter Skinner (Mitch Pileggi) had been detained by a United States Senate committee seeking to uncover Mulder's whereabouts when they cannot reveal his current location, but the committee was adjourned upon Mulder's arrival. The agents attempt to track down Peskow, following a trail of murders in Boca Raton, Florida, as well as locating a stolen truck. However, the assassin is able to outwit the agents, and destroys the last of the oil-containing rocks seen in the previous episode in an oil-well explosion after Mulder fails to remove the bomb. He returns to Russia, where it is revealed that he had been hired for this task by Krycek.