Doctors in Martinsburg, West Virginia, attend to a woman, Amanda Nelligan, as she goes into labor. When a nurse asks about the baby's father, Amanda tells her that the child's father is from another planet. When the baby is delivered, those in attendance gasp in horror when they see that the crying newborn has a four-inch tail.

Word of the strange birth is spread by newspaper tabloids, and Mulder and Scully travel to Martinsburg to investigate. During the drive, Mulder notes that five babies were born with vestigial tails over the course of three months -- all within a city of less than 15,000 people. Scully hypothesizes that the abnormalities may be attributable to ground water contamination or prescription drug interaction.

At the hospital, Amanda, a Star Wars fan, tells the agents the baby's father is Luke Skywalker. She recounts the tale of how Luke Skywalker came to her home and romanced her. With the assistance of a Health Department doctor, Scully determines that each of the five children born with tails share the same father. Mulder notes that all five women also shared the same fertility specialist, Dr. Alton Pugh.

When the agents arrive at Pugh's office, they discover several angry couples confronting the doctor about the births. As Mulder looks around the doctor's office, he encounters a janitor, Eddie Van Blundht, fixing a leaky sink inside an exam room and notices above his sagging pants a feint, roughly triangular scar at the base of the man's spine. Eddie tries to run, but is quickly apprehended and taken into custody. All of the women present swear that the only person they have had sex with has been their husbands. Later, a paternity test reveals that Eddie is the father of all five children. Scully hypothesizes that he may have used the tranquilizer Rohypnol to incapacitate his victims. Eddie, a homely man, responds defensively at the inference that the only reason a woman would have sex with him is if she was coerced.

While a sheriff's deputy is filling out his arrest record, Eddie suddenly changes form, metamorphosing into an exact duplicate of the deputy. Eddie strikes the astonished deputy on the head and makes his getaway. After questioning the deputy, Mulder concludes that Eddie impregnated four of the five women by making himself look like their husbands. The fifth woman, Amanda, was tricked into believing she was making love to Luke Skywalker.

The agents travel to the Van Blundht residence, where they meet Edward Sr. He claims he once performed as "Eddie the Monkey Man" and still has his tail intact. As they talk to him they realize the old man is, in fact, Eddie Jr. in a different form. Eddie suddenly runs out of the house and disappears into the neighborhood. After searching the house they discover the mummified body of the real Edward Sr. inside the attic. Scully's autopsy reveals an anomalous muscular structure beneath the surface of the corpse's skin. Essentially the man's skin was also a muscle. Mulder speculates that if Eddie inherited this same trait, it would explain his ability to change form. Meanwhile, Eddie metamorphoses once again -- this time into a duplicate Mulder. The real Mulder tracks him to a locker room at the hospital where Amanda Nelligan is staying. Suddenly, Eddie bursts through the ceiling, knocking him to the ground. He ties up the real Mulder and locks him inside the hospital's boiler room.

Pretending to be Mulder, Eddie returns to Washington with Scully, files a report with Assistant Director Skinner and closes the case. Later, the fake Mulder shows up at Scully's apartment with a bottle of wine. After spending the evening having relaxing conversation and relating in a personal rather than professional manner, Scully tells the fake Mulder, "I really feel like I'm seeing a different side of you tonight." The fake Mulder leans in close, and prepares to give her a kiss. Just as their lips are about to meet, the real Mulder kicks in the front door and interrupts them. Eddie slumps back on the couch, thwarted, and transforms back to his normal appearance and is taken into custody.

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