Agts. Mulder and Scully traveled to Pittsburgh, Pennsylvania, to investigate the apparent theft of a body from the morgue of the Monongahela Medical Center. The body was that of Leonard M. Betts, 34, an Emergency Medical Technician (EMT) for that hospital. Betts had died the night before when the ambulance in which he had been working had crashed. In the crash, Betts’s head had been severed from his body. Later that night, the morgue attendant heard banging noises in the freezer area. When the attendant investigated the noises, he discovered Betts’s severed head on the floor. The attendant was then struck from behind and knocked unconscious. His uniform was stolen.

Agts. Mulder and Scully examined the morgue drawer that had contained Betts’s body. They reviewed Betts’s records and learned that Betts had frequently been highly commended for excellent work as an EMT. No alarms had gone off during the attack on the attendant, indicating that the morgue had not been forcibly entered. Agt. Scully suggested that the attack on the attendant was part of a cover-up for body-snatching. The agents then reviewed a still from video-camera surveillance of the freezer area recorded on the previous night. In the still, they observed the attendant's clothes on a man, but apparent lens flare had obscured the perpetrator’s head. Agts. Mulder and Scully then searched the medical waste area, which Agt. Scully had suggested as a possible place of concealment for the stolen body. There they found Betts’s head.

While Agt. Mulder proceeded to Betts’s apartment, Agt. Scully examined Betts’s head. She observed an absence of rigor mortis and a lack of cloudiness in Betts’s corneas, noting that such observations were inconsistent with the occurrence of Betts’s death 11 hours earlier. Agt. Scully was then startled by the opening and closing of Betts’s eyes and mouth.

In the bathroom of Betts’s apartment, Agt. Mulder discovered blood stains and the morgue attendant’s uniform. The bathtub was full of a dark liquid. Agt. Mulder also discovered bottles of iodine in a cabinet. Agt. Scully phoned Agt. Mulder and informed him that the quality of Betts’s PET scan was degraded in the same manner as the surveillance video. The only possible cause of such degradation would be a high degree of radiation, but Agt. Scully could not discover any source of such radiation. When asked by Agt. Mulder for the results of her examination of Betts's head. Agt. Scully replied that there had been an unusual degree of galvanic response from Betts's head, which she described as having blinked at her. Agt. Mulder suggested that Betts’s head might not be fully dead or fully alive. He suggested that Betts had been in the apartment without his head. After Agt. Mulder left the apartment, Betts arose from beneath the surface of the liquid in the tub. His face appeared to be in an embryonic state.

In the parking lot of Monongahela Medical Center, Agt. Mulder interviewed Michelle Wilkes, the EMT who had been driving the ambulance from which Betts had been thrown during the crash. Wilkes described Betts as having had no friends and as having possessed a highly unusual degree of medical intuition. According to Wilkes, Betts could diagnose a person as having cancer merely by observation. Wilkes also stated that Betts never became ill and had never been injured.

In the presence of Agt. Mulder, Agt. Scully performed a biopolymerization of Betts’s head in order to harden it for slicing. A hospital pathologist then took an anterior slice from Betts’s head. Observing the slice on a video feed from a microscope, Agt. Scully observed that Betts’s brain was riddled with cancer. The pathologist remarked that most individuals would have died long before reaching such an extremely metastatic stage of cancer. The pathologist suggested that the image had been distorted by the polymerization process. Agt. Mulder suggested that the image might not be distorted and requested a slice to go.

While unloading a patient in the parking lot of Monongahela Medical Center, Michelle Wilkes overheard a two-way radio broadcast in which she recognized Betts, serving as a dispatcher at Allegheny Catholic Hospital, giving useful advice to an EMT.

Agts. Mulder and Scully allowed Agt. Mulder’s friend Dr. Charles Burks, of the University of Maryland at College Park, to examine a slice of Betts’s head in his lab. Dr. Burks described his equipment as being capable of photographing "coronal discharge," or life force. Agt. Scully evinced a high degree of skepticism regarding the procedure. Dr. Burks’s photograph of the slice showed not only the head but also a neck and shoulders. Dr. Burks was surprised to learn from Agt. Mulder that the head had been decapitated. Leaving Dr. Burks’s lab, Agt. Mulder suggested that Betts had regenerated a head in the manner of a reptile or amphibian. He noted that the type of iodine he had found in Betts's apartment as used by researchers to aid regeneration in such animals. Agt. Scully reminded Agt. Mulder that regeneration is not possible in mammals. Agt. Scully then received a phone call informing her that Betts’s fingerprints had been traced not only to Betts but also to a man identified as Albert Tanner. The agents concluded that Betts had an alter ego. Unlike Betts, Albert Tanner had a living relative, his mother Elaine Tanner.

At Elaine Tanner’s apartment, Agts. Mulder and Scully observed a photograph of a man they recognized as Leonard Betts. Elaine Tanner identified the man as her son Albert, who had died six years earlier in a car accident.

Meanwhile, in the parking lot at Allegheny Catholic Hospital, Michelle Wilkes found Leonard Betts working as an EMT. She approached him. Leonard Betts expressed his wish that she had not found him. While the two embraced, Betts injected her with an automatic syringe. Wilkes convulsed, collapsed, and died. Her collapse was witnessed by a security guard, who pursued Betts and soon subdued him. The security guard left Betts handcuffed to a car and went for assistance. Betts soon escaped by tearing off his thumb. When the guard returned, there remained only a severed thumb and the handcuff stained with blood.

The next morning, Agts. Mulder and Scully examined the thumb and the cuff at the scene. They bagged the thumb for evidence. Agt. Scully identified the substance with which Michelle Wilkes had been injected as potassium chloride. Citing the punctual equilibrium theory of evolution, Agt. Mulder suggested that Leonard Betts would soon regenerate the thumb. Agt. Scully observed that if Agt. Mulder were correct, then Leonard Betts would be so radically evolved as to be inhuman. In the trunk of Betts’s Dodge Dart, the agents discovered bagged medical waste consisting of various cancerous tumors. Agt. Mulder suggested that the tumors served Betts as food. He went on to hypothesize that natural selection might be incorporating cancer into human genetic makeup, and that Betts worked as an EMT in order to gain access to cancer wards. The agents then noted that the Dodge Dart was registered to Elaine Tanner.

At Elaine Tanner’s home, the agents led officers on a search of the premises. They again interrogated Elaine Tanner, who insisted that her son would not kill without a reason. She further stated that God had put her son here for a purpose. Agt. Mulder discovered in Elaine Tanner’s house a key to a locker at the U-Keep-It storage facility.

Meanwhile in a shabby bar, Betts observed a coughing smoker identified as John Gillnitz. Betts’s thumb was then in the process of regenerating. Betts followed Gillnitz outside to Gillnitz’s car. He apologized to Gillnitz, telling Gillnitz that Gillnitz had something Betts needed. He then attacked Gillnitz with a surgical knife. In a dark room, Betts appeared to disgorge through his mouth another person identical to himself.

Agts. Mulder and Scully arrived at the U-Keep-It storage facility, where they discovered the body of Gillnitz. Betts attempted to escape in Gillnitz’s car. When the agents fired on the car, the vehicle exploded.

In the pathology lab, Agt. Scully examined Gillnitz’s body. She observed that his left lung had been skillfully removed. Agt. Mulder expressed his certainty that Gillnitz had lung cancer and suggested that Leonard Betts’s need to consume cancerous tumors allowed him to sense the presence of such tumors in others. Agt. Scully insisted that Betts was dead. Agt. Mulder argued that Betts and Albert Tanner were the same man. That night, the agents opened the exhumed casket of Albert Tanner and discovered a well-preserved body. They compared it to the body of Leonard Betts and observed the bodies to be identical. Agt. Scully suggested that the two men were monozygotic twins and reminded Agt. Mulder that asexual reproduction would in fact be a pre-evolutionary ability in a mammal. Agt. Mulder argued that the fiery crash that had killed Albert Tanner was a ruse and that Leonard Betts was still at large.

At her home, Elaine Tanner washed Betts with iodine. Betts appeared to be emerging from an embryonic state. He also appeared weak and tired. Elaine Tanner urged Betts to restore his strength. Over his feeble protest, she insisted that it is a mother’s duty to provide.

Agts. Mulder and Scully arrived outside Elaine Tanner’s home. Before they could enter, an ambulance also arrived. The agents briefly held the EMT’s at gunpoint. The EMT’s protested that they had been summoned regarding an elderly woman with massive chest trauma. The agents then entered house, their guns still drawn. Agt. Scully observed Elaine Tanner with an open surgical wound. Agt. Mulder called for the EMT’s. Agt. Mulder searched the house and neighborhood but failed to locate Betts and returned to the Tanner home, where Agt. Scully informed him that Elaine Tanner was likely to survive and might be able to reveal Betts’s whereabouts. Agt. Mulder instructed the police to cordon the area. Agt. Scully accompanied Elaine Tanner in the ambulance. On arriving at the emergency room at Allegheny Catholic Hospital, Agt. Scully phoned Agt. Mulder, who was leading officers on a house-to-house search of the Tanner neighborhood, and informed him that Elaine Tanner would not be likely to give them information that night. While she was on the phone with Agt. Mulder, Agt. Scully noted iodine dripping from the roof of the ambulance. Agt. Scully summoned Agt. Mulder to the hospital. She climbed the rungs on the side of the ambulance but found nothing on its roof. Agt. Scully was then thrown by Betts into the ambulance. Betts apologized to Agt. Scully and informed her that she had something he needed. He then produced a surgical knife. During the struggle, Agt. Scully used the ambulance’s fibrillation panels to shock Betts. The surge jolted Betts out of the ambulance and into the parking lot, where he lay unconscious. Betts was then taken to the emergency room, where he later died.

Agt. Mulder caught up with Agt. Scully in the parking lot. He informed her that Betts’s mother had a cancerous tumor in her neck muscles but would survive her wounds. Agt. Mulder praised Agt. Scully for her work on the case. Agt. Scully distractedly asked to be taken home. Late that night, Agt. Scully awakened to find her nose inexplicably bleeding.

Though no positive explanation for the events of the case could be determined, the case was closed after the death of Leonard Betts.