Mulder tells Scully about a series of identical attacks that occurred over a period of twenty-eight years in almost every state in America. One of the most recent involved Jerald Glazebrook, who performed as The Alligator Man in carnivals and circuses. The agents travel to Gibsonton, Florida, where they attend Glazebrook's funeral. They soon realize that most of the town's residents suffer from physical deformities, and that most work the side-show circuit. When Mulder takes an interest in a drawing found on a menu, Sheriff Hamilton directs the agents to Hepcat Helm, who operates and designs exhibits for a carnival fun house. Hepcat explains that the drawing on the menu depicts the Fiji Mermaid, an exhibit that P.T. Barnum called "the genuine fake" because it fooled no one. Mulder theorizes that the Fiji Mermaid may, in fact, be quite real.

The agents take rooms at a local trailer park run by a midget named Mr. Nutt. Lanny, a Siamese twin, helps the agents with their luggage. Later that night, while working alone in his workshop, Hepcat is murdered by a strange creature. A preliminary examination of Hepcat's body produces more questions than answers.

Later, the agents speak with Dr. Blockhead, who claims he has trained his body to accept acute pain. He demonstrates his abilities by driving a nail up into his nostril. Mulder aids Blockhead by removing the nail with a pair of pliers. Before the agents leave, they spot The Conundrum, a man who eats live animals and just about anything else he can fit in his mouth. Blockhead describes his friend as a "geek." Mulder keeps the nail taken from Blockhead's nose and has the blood analyzed, hoping it will match blood found at Hepcat's workshop. The blood samples match, but both turn out to be O positive.

Mulder tells Scully that he ran background checks on some of the local side-show performers. His most interesting discovery reveals that Sheriff Hamilton once performed as Jim-Jim, the Dog-Faced Boy. But after surveying Hamilton, the agents conclude he is not a suspect.

Soon afterward, Mr. Nutt becomes the killer's latest victim. The agents conclude that the murderer is so physically small that he was able to climb through the doggie door in Nutt's trailer. When a drunken Lanny learns of Nutt's death, he flies into a rage. He is arrested by Hamilton and transported to jail.

The agents approach Dr. Blockhead for further questioning. But Blockhead, a magician and escape artist, easily eludes them. Sheriff Hamilton detains Blockhead by grabbing hold of the fish hooks which pierce his skin. When the agents, Hamilton, and Blockhead arrive at the jail, they notice that Lanny is in pain. Scully realizes that Lanny's twin brother, Leonard, is capable of extracting himself from his brother's body. Lanny admits that Leonard is seeking another brother with whom he can co-join. The agents track Leonard into the fun house, but their efforts to capture him prove futile.

Later, Blockhead laments that due to advances in genetic engineering, people with severe abnormalities will not exist. Everyone in the future, he fears, will look like Mulder. Dr. Blockhead and the Conundrum, who suffers from a swollen stomach, pack up their belongings and prepare to leave town. As their vehicle hits the open road, Mulder and Scully look questionably at each other, perhaps realizing that the Conundrum may have eaten Leonard.