S4E19 · Synchrony

In Cambridge, Massachusetts, MIT cryogenics researchers Jason Nichols (Joseph Fuqua) and Lucas Menand (Jed Rees) become embroiled in an argument as they walk down a city street. They are approached by an old man (Michael Fairman), who warns Menand that he will be killed by a bus at 11:46 p.m. that evening, but Menand ignores him. After the man is arrested by campus security, his apparent prophecy is proven true when Jason tries, but fails, to save Menand, who is promptly run over by a bus and killed at exactly 11:46 p.m.

Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) investigate the case, learning that Jason was taken into custody after the bus driver told police that he (Jason) pushed Menand into the path of his vehicle. However, Jason tells authorities that he was actually trying to save Menand. The security guard who arrested the old man is found frozen to death after exposure to a chemical refrigerant. Mulder interviews Jason, who explains Menand threatened to go public with a claim that Jason had falsified data on a research paper.

The old man kills Dr. Yonechi (Hiro Kanagawa), a Japanese researcher on the topic of biological vitrification for cryopreservation, by pricking him with a metallic stylus, introducing an unknown chemical into his body. The agents approach Nichols' girlfriend and colleague, Lisa Ianelli (Susan Lee Hoffman), who recognizes the chemical compound as a rapid freezing agent that Jason had been engineering for years. However, she claims that the compound has not yet been invented and that if Yonechi was injected with the chemical, he may not be dead. With Lisa's help, Scully and a team of medical personnel successfully resuscitate Yonechi, only for his body temperature to rapidly increase until he bursts into flames.

When getting off the bus, Lisa finds that she is being followed by the old man, who shows the weapon but runs off; when asked by Scully and Mulder about him, she reveals that Jason is covering for her in faking data related to a potential grant.

Police receive a tip that the old man is living at a nearby hotel. Inside the old man's room, the agents discover a faded color photograph picturing Jason, Yonechi and Lisa toasting champagne glasses in the cryonics laboratory. Mulder deduces from the picture that the old man is a time traveller who is attempting to alter that future, and that he is none other than Jason Nichols.

Lisa locates the elderly man and confronts him; however, he injects her with the chemical after explaining that Lisa will be responsible for the coming future. Scully successfully resuscitates Lisa. Jason confronts his elderly self in the computer mainframe room at the cryogenic laboratory, where the old man has erased all of Jason's files from the mainframe. The old man tells Jason that the success of their research made time travel possible, but also plunged the world into chaos. Jason lunges at the old man, choking him as he demands to travel back to save Lisa. Mulder arrives with the news that Lisa has been saved, and the elder Jason wraps his arms around his younger self and spontaneously bursts into flames, the fire consuming them both.

As Lisa is loaded on to an ambulance to receive further treatment, Scully breaks the news to her that Jason died in the mainframe room fire. Mulder then reminds Scully of a statement in her thesis in which she asserts that "... although multidimensionality suggests infinite outcomes in an infinite number of universes, each universe can produce only one outcome," which indicates that, despite Jason Nichols effectively erasing himself from the future, the rapid freezing agent compound will be discovered by someone in the current universe, resulting in the eventual discovery of time travel.

Later, Lisa sets to work at the cryonics laboratory, attempting to reconstruct the chemical compound.