Mulder and Scully investigate the gruesome murder of Roger Crockett, a homeless man who was attacked and partially eaten in a wooded area outside Atlantic City. Mulder believes the death is related to a similar incident that occurred in New Jersey in 1947, in which a motorist was attacked and eaten by a half-human creature as he attempted to fix a flat tire.

The agents travel to the Atlantic City morgue to gather more information. They clash with Detective Thompson, the local police official in charge of the investigation. Unphased, Mulder decides to perform an investigation on his own. He interviews Ranger Brullet, who discovered the body, and a homeless man named Jack, who knew the victim. Both believe Crockett was killed by The Jersey Devil, a legendary half-human creature that roams the New Jersey countryside. Meanwhile, Scully attends her godson's birthday party, where she meets Rob, a handsome divorcee.

Mulder stakes out a dark alley near the woods where Jack claims the Devil has been prowling. That night, the creature makes an appearance, but before Mulder gets a good look at it, the police arrive and arrest him, mistaking him at first for a vagrant.

At the jail, Mulder again clashes with Thompson, accusing him of keeping the creature a secret so as not to upset tourism in Atlantic City. Scully bails Mulder out of jail and the duo travel to the University of Maryland, where Dr. Roger Diamond affirms Mulder's theory that a human being might at birth revert to its most primal instincts-devolving into a carnivorous Neanderthal. Scully, meanwhile, goes on a date with Rob.

Ranger Brullet finds another body in the backwoods, but this time the corpse appears to be half-animal. Mulder theorizes that two such creatures may have roamed the countryside. He and Scully comb an abandoned building for the Devil as the police fan out to capture them. Mulder manages to allude police long enough to locate the second creature, which turns out to be female. She suddenly attacks and wounds Mulder and escapes into the woods.

Police track the creature and kill it, despite Mulder's plea to take it alive. An autopsy report confirms the female creature recently gave birth, leading Mulder to conclude it came to the city in search of food scraps for its young after its mate died. Some time later, a feral child peeks out from a small den as hikers pass by.