Mulder and Scully delve into an old case involving a mysterious player in the Negro Baseball League with an astonishing batting average.

On a beautiful Saturday afternoon, Mulder and Scully are in the office. Mulder is looking through old newspapers for evidence of flying saucers and finds an article mentioning Arthur Dales in Roswell, New Mexico, in a story relating to baseball. Scully has little time for baseball and accuses Mulder of being more interested in baseball than in work. Mulder goes to Dales' home and finds another man there, who says he is the former FBI agent's brother, also named Arthur Dales. (He says their parents were not very creative in selecting names.) Mulder tells Dales that the newspaper picture also appears to show an alien bounty hunter. Mulder has to validate himself by answering baseball trivia questions, but Dales eventually invites Mulder in. Dales asks the philosophical question of whether a passion can change a man, really shapeshift and transform him.

On June 29, 1947, Dales was a police officer in Roswell and was assigned to protect Negro baseball star Josh Exley from racist threats that the game of baseball should be kept white. Exley plays for an all-Negro team called the Roswell Grays. Dales ends up travelling on the team bus, and at night, in a lightning storm, Exley's reflection in the bus window appears to be the image of a grey alien. The next day, during a game, Exley is hit on the head with a ball and knocked out. He murmurs words in an unintelligible language and claims to be from Macon, Georgia. Dales finds green acid on the glove used to cushion Exley's head and sends it off to be tested. Dales calls the Macom police and learns that Josh Exley was a six year old child that disappeared about the time Exley arrived in Roswell. The Macon police officer Dales talks with is really a bounty hunter. Exley denies to Dales that he was ever in Macon, but Dales thinks Exley "tanked" the game that day because American League scouts were there and he did not want to be recruited into the major leagues, because Exley has a secret.

That night, Dales sees a grey alien in Exley's room, wearing a baseball cap and holding a bat. Dales faints at the sight, but the alien wakes him up and tells him he is Exley - this is his real face. Exley morphs into a pretty girl to prove his claim. The next day they talk more. Exley says his people guard their privacy and don't allow intermingling, but when he discovered baseball he immediately fell in love. The bounty hunter arrives in Roswell, looking for Exley. Posing as Exley, he goes to the lab and kills the lab technician who was testing the acid on the glove. Dales finds Exley and warns him that a witness claims he killed a man. Exley says he talked "with his family" and they want him to come home. He leaves, supposedly to go back home, and several "men in black" question Dales.

Exley actually, however, returns to the baseball team and plays a night game. After a home run, several men in the white sheets of the Ku Klux Klan and carrying guns ride up on horses. Exley has gotten too good and the KKK wants to keep the game of baseball white-only. They threaten Exley, but the white baseball team overcomes the Klansmen, one of whom, knocked unconscious, is seen to be a grey alien. Everyone else runs off and Exley confronts the bounty hunter, who quickly awakes. The bounty hunter says he warned Exley and tells him to show his true face so he can die with honour. Exley does not morph. Dales drives up as Exley is killed. The bounty hunter escapes and Dales concentrates on Exley, who is dying. Exley warns Dales that his blood is like acid, but Dales finds only red blood. As Dales had suggested to Mulder, Exley apparently loved the game of baseball so much that it transformed him into a human, but Mulder can't figure out if the story is metaphorical.

Late at night, Scully arrives at a baseball diamond where Mulder is hitting baseballs thrown by a machine. Scully claims she was called by "Fox Mantle" for an early to late birthday present. Mulder shows Scully hot to use a bat, wrapping his arms closely around her. He jokes about leaving behind a career in medicine to hunt aliens and reveal a global conspiracy, but Scully says "shut up, Mulder. I'm playing baseball," with a big grin on her face.