S8E17 · Empedocles

In New Orleans, mild-mannered white-collar worker Jeb Larold Dukes (Jay Underwood) is fired from his job. After he leaves the office, Jeb witnesses a car chase which ends in a fiery crash. A burning figure that only Jeb can see emerges from the wreck and seems to merge with him. Jeb, now possessed by the being, returns to his office to massacre his boss and co-workers.

Monica Reyes (Annabeth Gish) arrives at the crime scene and meets NOPD detective Franklin Potter, who has called her in out of his belief that the murders are related to satanism. Reyes merely answers the detective that it was not devil worship, and that the killer probably was under stress. As she is leaving, she witnesses one of the bodies carbonize into a charred corpse in front of her, only to have it revert to normal.

Fox Mulder (David Duchovny) takes a pregnant Dana Scully (Gillian Anderson) to the hospital when she doubles over in pain. There, Reyes phones him to ask about the case, but Mulder cedes to the authority of John Doggett (Robert Patrick), who is formally running the X-Files. However, Reyes says she can't call Doggett since it is about him. Meanwhile, Jeb is in a hotel room in Georgia trying to shoot himself in the head. Suddenly, he notices something wrong with his face; he claws at his face, peeling away the skin to reveal burning embers underneath.

Mulder and Reyes meet in an FBI records room, where she divulges that she was one of the agents called in to investigate the murder of Doggett's son Luke years previously. They never found the killer, and when they discovered Luke's body, both Reyes and Doggett saw it transform into ashes instantly, something Doggett has spent the last few years convincing himself he did not see. Reyes believes it was a psychic clue, and now with the other crime scene, she has seen it again.

Doggett soon attacks Mulder for looking into his son's case, but Reyes explains why they are investigating. She reveals that Bob Harvey, a suspect in Luke's murder, died in the car crash in New Orleans. Reyes stresses her vision's importance, but Doggett dismisses it. Reyes then visits Jeb's sister, Katha, and learns he did not know Bob Harvey. While Reyes is there, Katha receives a call from Jeb, but tells him to call later. As Jeb leaves the phone booth, he kills a female motorist. Meanwhile, Doggett learns from Scully that she has suffered a placental abruption. He asks her what made her start to believe in the paranormal. She says she realized she was "afraid to believe." Scully later tells Mulder to keep trying to help Doggett.

As Doggett approaches the body of Jeb's latest victim, he has a flashback of when he found Luke's body. Doggett storms off, but Reyes refuses to let it go, finally revealing her theory: she believes that the boy's murder was part of a "thread of evil," an interconnected series of terrible events. Meanwhile, Katha returns home with her daughter and finds Jeb there. He insists that it wasn't him who committed the murder, and pleads to his sister for help. Katha calls the agents and tries to separate her daughter from Jeb, but he realizes what she is planning and uses his niece as a hostage. Before Jeb can shoot Doggett, Reyes incapacitates him with a round to the throat.

Doggett finally embraces the memory of his vision. Mulder tells him that when he worked in violent crimes and saw the horrible things people did, he began to think of evil like a disease that infected people. Some lack immunity to this disease of evil because of some tragedy in their lives. Jeb dies while Reyes and Katha are in the room. After the doctors leave, Katha's eyes glow, much like Jeb's. She hits Reyes over the head with an oxygen tank and takes her gun. Before she can execute Reyes for killing her brother, Doggett appears and wrestles her to the ground.

At the end Doggett looks in on Katha seeming to be drugged laying in a hospital bed.