When Mulder had just joined the FBI, he shot John Barnett (David Peterson), though not before he killed another agent during a robbery.
Barnett was taken alive, though instead of dying, the prison doctor, Dr. Joe Ridley (Robert Mossley) used him for certain experiments revolving around attempted to discover a cure for progeira.
Originally, Ridley was a doctor of renown, till he tested his theories about progeira on people, despite the resistance of the medical establishment.
This resulted in him losing his medical license.
His experiments, udesigned to worked with no one but Barnett, and resulted in him aging backward.
It goes without saying that the government is bargaining with Barnett behind the scenes, according to Deep Throat (Jerry Hardin) ,in an attempt to get the research notes that Barnett had stolen from Dr. Ridley.
This episode is probably one of the most effective of the stand-alone episodes.