"Young at heart" .............

Mulder and Scully are asked by Special Agent Reggie Purdue to aid in the investigation of a jewelry store robbery. Mulder realizes his old friend Purdue requested his assistance with the case because of a note found at the crime scene that reads: "Fox can't guard the chicken coop." Mulder explains to Scully that his first FBI case involved an armed robber named John Barnett, who left notes that taunted Mulder about the crime spree. Barnett was eventually captured and sentenced to life in prison, where he died four years earlier. Mulder is disquieted when a lab technician analyzes the note and concludes it was most likely written by Barnett within the last 48 hours.

Meanwhile, Scully and Purdue review video surveillance footage of Barnett's capture by the FBI years earlier. In the footage, Mulder had the opportunity to kill Barnett, but hesitated because of FBI procedures regarding hostage situations. That hesitation allowed Barnett time to kill an armored car driver and an FBI agent. Mulder returned fire, wounding Barnett. Mulder never forgave himself for the error. During the trial, Barnett swore he would "get" Mulder.

Scully and Mulder visit Barnett's old prison cell mate, Joe Crandall, who swears Barnett was alive on the night he supposedly died. He also claims he saw a prison physician, Dr. Ridley, amputate Barnett's right hand. Mulder phones Purdue with the news, but during the conversation Purdue is attacked and strangled. Mulder and Scully rush to the scene but arrive too late. Another of Barnett's notes is found on Purdue's body.

The agents pay a visit to the National Institute of Health, where they meet Dr. Austin. He explains that Dr. Ridley experimented on young children afflicted with a disease called Progeria, which accelerates the aging process and causes premature death. Ridley lost his license to practice medicine and performed age-reversal experiments in secret. Mulder becomes convinced that Barnett was one of Ridley's guinea pigs, and may now sport the physique of a younger man.

Mulder and Scully are approached by the now-ailing Dr. Ridley, who tells them that Barnett is the last of his patients who is still alive. Ridley explains that he used a different technique on Barnett. He grafted the cells of a salamander onto his arm. Those cells grew and mutated into Barrett's new hand. Mulder learns from Deep Throat that Barnett stole Ridley's secrets on age-reversal and now wishes to trade the medical information to the U.S. government in exchange for immunity.

Realizing that Scully is Barnett's next intended victim, Mulder and his fellow agents stake out a recital hall where Scully is to meet a friend. Barnett shoots Scully, but she is protected by a bullet proof vest. Barnett takes a cellist hostage, but Mulder sees his chance and fires, killing Barnett.