Albert Tanner was an EMT in Pittsburgh, Pennsylvania, who had the ability to regenerate lost body parts. Under the alias Leonard Betts, he served on an ambulance driven by Michelle Wilkes that was struck by a truck and Betts was decapitated. Wilkes made the arrangements for his body, saying that Betts had no real friends and no living family. Mulder and Scully were brought in to investigate and discovered that Betts' body had been stolen from the hospital. Scully was about to perform an autopsy on the severed head, but was too unnerved by the residual electrical activity that caused it to blink and open its mouth.
Due to strange security video feeds and medical scans, Mulder began to suppose that perhaps Betts' body was able to leave on its own accord and regenerate its severed head - an idea that Scully found absurd. Mulder began investigating Betts' home and discovered an entire bathtub full of iodine, which is used to facilitate tissue regeneration. The agents also followed a lead that led them to Elaine Tanner, whose son "Albert" had died in a car accident six years prior. Albert Tanner and Leonard Betts were clearly identical, yet she did not reveal any connection.
Further medical examination revealed that Betts' entire brain cavity consisted entirely of cancerous material, so much that there was no way he could have lived in that state. Yet Mulder saw this as vindication, because cancer, by its very nature, is capable of astonishing regeneration and growth. It was also of note because Wilkes said that Betts had an uncanny ability to diagnose cancer patients simply by looking at them. While driving the ambulance, Wilkes overheard a radio conversation where one of the speakers sounded just like Betts. She sought him out at another hospital and was shocked to see him alive. Betts embraced her and apologized that she should discover his secret. To protect his identity, he then killed her with a lethal injection of electrolytes. A security guard witnessed this and chased Betts down and handcuffed him to a car. While the guard went to check on Wilkes, Betts ripped off his own thumb and escaped.
While investigating the scene, Mulder and Scully discovered in Betts' car a pile of medical waste, mostly cancerous tissue. Mulder theorized that he was consuming the cancer to provide for his regenerative ability. Betts would kill again the next day. Sure enough, it was a man with lung cancer, to whom Tanner said, "You have something I need!" Mulder found a key to a storage locker in Elaine Tanner's home and went there with Scully, where they found Betts and the dead man with cancer, whose cancerous lung had been carefully and skillfully removed. Betts attempted to run them down with his car, but the agents shot at it and it exploded. Scully began to accept Mulder's theory more and more, and the casket of Albert Tanner was exhumed, revealing a burnt corpse nearly identical to that of Betts'. Mulder's regeneration theory was now so extreme as to suggest that even a trace amount of tissue could regenerate into an entirely new human.
Events were now coming to a head and the agents began a stakeout of Elaine Tanner's residence. Soon, an ambulance arrived and Mulder realized that Betts was in the house with Tanner, his mother. They raced in and discovered that a cancerous growth had been removed from her, undoubtedly by Betts. Mulder began a search of the area, and Scully accompanied Tanner to the hospital in the ambulance. Upon their arrival, Scully discovered dripping iodine and came to the realization that Betts had somehow traveled with them. Drawing her pistol, Scully was ready when Betts attacked. She was able to fend him off, but he said, "You have something I need!", indicating that he detected cancer in Scully, who was not yet aware that she suffered from the disease.. As he lunged at her again, Scully grabbed a set of defibrillators and shocked him in his head, killing him once and for all.