FBI Agent Tom Colton describes to Scully a series of murders he believes were committed by a serial killer. The victims were all murdered inside locked or secured rooms. Each had their liver ripped out of their body.
     Despite Colton's discomfort, Mulder is brought into the case. Mulder finds strange humanoid fingerprints at the high-security office building where businessman George Usher was murdered. Mulder links the prints to similar killings that occurred in the United States every thirty years, going back to at least 1903. In each occurrence, five victims had their livers torn out.
     Instead of combining forces, Scully and Mulder decide to take different avenues of investigating the case: Scully the traditional; Mulder the unorthodox. At the scene of the Usher murder, Mulder assures Scully that the suspect would not return, having already beaten the security system. He thinks the killer would seek a new challenge. But as they talk, Mulder discovers someone crawling inside an air duct. The suspect, an Animal Control officer named Eugene Tooms, is taken into custody. Tooms passes a polygraph test... except for those questions posed by Mulder regarding his age.
     Mulder suspects Tooms is the killer. Using a computer, he manipulates the image of Tooms' fingerprints, elongating them. They are compared to the prints from 1933. The prints match. Scully is intrigued and re-teams with Mulder on the case.
     Tooms attacks and kills another victim guarded by high-tech security equipment. Tooms disappears; Scully and Mulder find that his apartment was never lived in. Scully tracks down Frank Briggs, the sheriff who investigated the five murders that occurred in 1933. Briggs describes how he tracked the murders until the present day, never able to prove his belief that Tooms was the killer.
     The agents locate an abandoned building that Tooms once listed as his address. Inside they discover a small room containing "trophies" taken from each crime scene throughout the century. Mulder speculates that Tooms may be a genetic mutation that hibernates for decades and eats human livers to provide sustenance. As they leave the scene, Tooms procures Scully's necklace.
     Scully arranges a stake-out of the abandoned building, but Colton calls it off without informing Mulder. Mulder arrives at the building, and finding no undercover agents, ventures inside. He finds Scully's necklace placed with the other trophies.
     Mulder races to Scully's apartment and finds her grappling with Tooms. Mulder draws his weapon and, with Scully's help, manages to handcuff Tooms. He is taken to jail, where he begins assembling a new "nest."