While performing a random search of a suitcase being carried by a man claiming to have diplomatic immunity, an airport customs agent is attacked by a black oil that congeals into tiny worms which penetrate his skin.

     An anonymous tipster warns Mulder about a right-wing militia organization planning a bombing which could be the next Oklahoma City. The informant turns out to be Alex Krycek, the traitor Mulder blames for his father's murder. Left trapped in an abandoned missile silo by the Cigarette-Smoking Man, Krycek says he was freed by a militia group during a salvage operation. Claiming he wants revenge on the Cigarette-Smoking Man, Krycek promises Mulder that he can help expose the Cigarette-Smoking Man and the Shadowy Syndicate. Despite his hatred for Krycek, Mulder reluctantly believes him.

     Krycek leads Mulder and Scully to intercept a Russian courier at the airport. The courier escapes, but the diplomatic pouch he is carrying is recovered. The pouch contains a four-billion-year-old rock of extraterrestrial origin. Dr. Sacks, a government exobiologist, drills into the rock. The same black worms which killed the customs agent emerge from the rock and attack him. Scully and Pendrell investigate this deadly enigma. Mulder's possession of the artifact alarms the Well-Manicured Man, who orders the Cigarette-Smoking Man to take care of the problem.

     Mulder seeks out his UN contact, Marita Covarrubias, who finds out the origin of the Russian courier's flight. Mulder learns the flight originated near Tunguska, Siberia and instantly recognizes the significance. In 1908, a fireball crashed to earth in Tunguska, igniting a series of cataclysmic explosions. It was the most massive and most mysterious event of its kind in history. Until now, no one had been able to discover what really happened. Maybe someone has finally found out the truth...

     Marita helps Mulder with credentials that will get him to Tunguska. At the last minute, Mulder discovers Krycek is fluent in Russian, and brings him along as a translator.

     Mulder and Krycek discover a Siberian Gulag located at the site of the crash -- its hard-laboring prisoners sentenced to mine the extraterrestrial rocks. Before they can learn any more, they are captured, beaten and jailed. Mulder realizes Krycek is in league with their captors. Krycek is freed, and Mulder is subjected to an inhuman medical experiment. Under the most primitive of conditions, Mulder and other prisoners are infected with the oily black worms, and injected on the left arm with an amber liquid.

Will Mulder survive?