S1E16 · Young at Heart

In 1989, Joe Crandall, an inmate at a federal penitentiary in Pennsylvania, hears screaming from the infirmary. Inside, he discovers the prison's doctor, Joe Ridley, amputating the hand of fellow inmate John Barnett. Ridley tells Crandall that Barnett is dead, threatening him with a scalpel. However, as Crandall leaves the room, he sees Barnett blinking.

Five years later, Fox Mulder is notified by his former FBI supervisor, Reggie Purdue, about a note from a jewelry store robbery mocking Mulder by name. Mulder recognizes the message as being from Barnett, a sociopathic multiple murderer whom he helped capture on his first case with the Bureau. Even though Barnett supposedly died in prison, the note bears his handwriting. Purdue shows Dana Scully a video of Barnett's capture, which shows that Mulder didn't fire on Barnett due to him having a hostage, per FBI procedure. Mulder's hesitancy allowed Barnett to kill both the hostage and a fellow agent.

Scully looks into Barnett's cause of death and discovers that despite it being listed as heart attack, he had no history of heart problems; he had been sent to the infirmary due to problems with his hand. Meanwhile, Barnett leaves Mulder another note in his car, along with photos of him and Scully. The agents visit the prison and meet Crandall, who recounts his experiences with Barnett and Ridley. Barnett makes taunting phone calls to Mulder and strangles Purdue with a disfigured hand. Scully looks into Ridley's past and finds that his medical license was revoked for performing illegal medical trials on children with progeria, a disease that causes premature aging. Mulder theorizes that Ridley's experiments helped him find a way to reverse the aging process.

Scully summons Mulder when Ridley suddenly appears at her apartment. He tells the agents that he succeeded in making Barnett age in reverse after replacing his hand using salamander cells. However, Barnett stole Ridley's government-sponsored research. Deep Throat meets with Mulder and confirms Ridley's story, saying that the government is negotiating with Barnett to purchase Ridley's work. Scully hears someone dialing into her answering machine and spots Barnett's fingerprint on it. After Barnett calls again, Mulder decides to set up a sting operation at the cello recital for a friend of Scully's, which Barnett learned about from her answering machine.

That night, FBI agents wait at the concert hall for Barnett's arrival. Barnett, who goes completely unseen due to his youthful appearance, poses as a piano tuner. He shoots Scully in the chest during the recital and flees, taking the cellist hostage. Mulder doesn't hesitate this time and fires at Barnett, fatally wounding him. Scully is revealed to have survived the attack, having worn a bulletproof vest. Despite the efforts of doctors and a mysterious CIA agent to resuscitate Barnett, he dies, and takes the secrets of Ridley's research to the grave. The episode ends with a close-up of a locker in a train station, implying the secrets are contained within and will one day be discovered.