S6E19 · The Unnatural

In 1947, a mixed group of black and white men play baseball in Roswell, New Mexico. A group of Ku Klux Klan (KKK) members arrive on horseback, seeking one of the players: Josh Exley (Jesse L. Martin), a talented black baseball player. Men from the team fight back against the KKK, and when the mask of the clan's leader is taken off, the leader is revealed to be an alien.

In 1999, FBI agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) look through Roswell newspapers from the 1940s. Mulder spots an article showing a young Arthur Dales (Fredric Lehne)—the original investigator of the X-Files division—Josh Exley, and the shape-shifting Alien Bounty Hunter (Brian Thompson). Mulder seeks out Dales in Washington D.C. but instead, meets Dales's brother (M. Emmet Walsh), who is also named Arthur.

In flashback, Dales tells Mulder about first meeting Exley in 1947. Dales, a member of the Roswell Police Department, has been assigned to protect a hesitant Exley. Dales travels with Exley and his teammates on their bus, and one night sees that the sleeping Exley is reflected in a window as an alien. The next day, during a game, Exley is hit by a pitch and starts making utterances in a strange language before returning to his senses. Afterwards, Dales notices that a mysterious green ooze appeared where Exley's bleeding head had rested.

Dales decides to investigate Exley's hometown of Macon, Georgia, and discovers that a boy with Exley's name had vanished about five years previously. That night at the hotel, Dales hears noises from Exley's room and breaks in, only to find Exley in his alien form. Exley tells Dales that he was forbidden from intermingling with the human race but fell in love with the game of baseball and remained on Earth. Exley took the form of a black man and played in the Negro leagues to avoid attracting attention. When major league scouts appear at one game, Exley deliberately performs poorly.

The Alien Bounty Hunter, who has been pursuing the renegade alien, takes Exley's form and murders a scientist who is investigating the green ooze that Dales found. Dales warns Exley that he is now wanted by the police, and Exley goes into hiding. The narrative returns to the events at the start of the episode. The KKK leader is revealed as the Alien Bounty Hunter, who has arrived to assassinate Exley. The Bounty Hunter demands that Exley revert to his true form before he dies. Exley refuses and the Bounty Hunter kills him. However, Exley bleeds red, human blood.