S5E15 · Travelers

In 1990 in Caledonia, Wisconsin, a man named Edward Skur is shot by a police officer during an eviction and the last word he speaks is "Mulder". Fox Mulder (David Duchovny), at this point working with the FBI 's Behavioral Science Unit, believes that the man may have had some connection to his father, Bill. Mulder discovers that Skur was reported to have died in 1952. He seeks out Arthur Dales (Darren McGavin), a retired FBI agent who investigated Skur in the 1950s. At first, Dales is reluctant to discuss the case and warns Mulder away. However, Mulder's threat of a subpoena persuades Dales to tell his story.

In a flashback to the 1950s, Dales (Fredric Lehne) and his partner Hayes Michel are sent to arrest Skur (Garret Dillahunt) for being a communist. When Dales is told that Skur hanged himself while in custody, he feels guilty and returns to Skur's house to apologize to his wife. While there, he sees Skur alive and tries to recapture him. In the following struggle, an appendage emerges from Skur's mouth. Skur is forced to flee when a neighbor interrupts the fight. Both Michel and Roy Cohn warn Dales to change his report about the attack. He does so, but feels guilty about it.

Later, Dales and Michel are called to investigate the death of a German doctor in Chevy Chase, Maryland. The local police deny calling them, and Dales finds a coaster for a bar with "come alone" written on the back. At the bar, Dales meets Bill Mulder, an agent from the State Department. Bill tells him that Skur was experimented on along with two other men, who later killed themselves; that Skur killed the German doctor in revenge for what was done to him and that he will kill Dales and Michel too because Skur believes they are part of the plot. Dales tries to warn Michel by telephone but is unable to prevent his murder. Dales tries to investigate his partner's death but Cohn covers it up.

A secretary at the FBI office, Dorothy Bahnsen (Jane Perry), helps Dales find a file that mentions Gissing, one of the two test subjects. Gissing's body is still in the morgue and Dales convinces the technician to cut open his body, where they find a strange creature has been sewn into Gissing's chest. Dales goes to Skur's wife and tells her what was done to her husband and that he wants to expose the experiments. Skur's wife goes down into the backyard bomb shelter to tell Skur, but he is overcome by his parasite and kills her.

Cohn brings Dales before FBI Director J. Edgar Hoover, who convinces him to help bring in Skur. They take Dales back to the bar to meet Skur, who attacks Dales. Bill and the other agent wait outside until they think Dales is dead before rushing in to find that he has handcuffed Skur and is still alive.

In 1990, Fox Mulder is dismayed to hear what his father was involved in. He asks Dales how Skur escaped and Dales speculates that someone might have helped Skur escape hoping that the truth of what was done to him may one day be revealed. The episode ends with a final flashback of Bill Mulder driving down a road with Skur and giving Skur the keys to the car and walking away.