Michele Wilkes, an Emergency Medical Technician, races an ambulance through the streets of Philadelphia as her highly-regarded partner, Leonard Betts, attends to a dying man. As the ambulance speeds through an intersection, it is broadsided by a truck. Wilkes survives, but she is horrified to discover Betts' decapitated body nearby.

     Mulder and Scully are brought into the case when Betts' headless corpse disappears from a hospital morgue. Scully theorizes that someone attempted to steal the body and sell it to an unscrupulous medical supplier. But when security camera footage shows an unidentified person with a strange distortion around the head area leaving the hospital. Mulder suspects something strange is going on--perhaps the body left by itself.

With Mulder's reluctant help, Scully fishes through the hospital's bio-disposal unit for the missing corpse. The agents recover the missing head, but the whereabouts of the body remains a mystery. As Scully prepares to perform an autopsy on the head, its eyes and mouth suddenly snap open.

     A tissue sample taken from Betts' head is analyzed by a pathologist who determines that Betts was riddled with cancer, so much that a normal human could have never lived that way. Mulder hypothesizes that Betts had somehow incorporated cancer into his body, and possessed unimaginable regenerative powers--perhaps the next step in Man's evolution. A fingerprint search reveals that Betts' real name is Albert Tanner. The agents interview his mother, Elaine, who claims that Albert died in an automobile accident six years earlier.

     As Wilkes is transporting another patient, she recognizes Betts' voice transmitting over the ambulance radio. She locates Betts working as an EMT at another hospital. As the two embrace, Betts sadly pulls out a syringe and injects Michele with a lethal substance. A security guard witnesses the murder and handcuffs Betts to his car. Betts slips out of the cuffs by tearing off his own thumb. When the thumb is found, Mulder guesses he tore it off because he knew he could grow another.

     When the agents search the trunk of Betts' car, they discover plastic bags filled with human tumors. Mulder postulates that Betts is ingesting the cancerous tissue in order to survive. They learn that the car is registered to Albert Tanner's mother.

     Weak and needing sustenance, Betts attacks and kills a heavy-smoking bearded man after he leaves a bar. Strengthened, Betts is then able to "gives birth" to a duplicate of himself.

     The agents discover a storage locker key at Mrs. Tanner's home. When they open the door to the locker, they find the smoking man's body, its left lung missing. Suddenly from within the locker, a car roars to life, and Betts attempts to run down Mulder and Scully. The agents dive for cover and open fire. The car bursts into flames, incinerating Betts.

     Though Mulder is convinced that something else is going on, Scully reminds her partner that Betts is quite dead and his not coming back to life, and the case is over. Nonetheless, Mulder exhumes the body of the man Mrs. Tanner claimed was killed in the automobile accident. Inside the casket is a corpse identical to Betts. Mulder concludes that the fiery crash was a ruse-and the real Leonard Betts is still alive.