Scully is told by her superiors that the investigation of all X-Files cases must adhere to proper FBI procedures. She counters that the extraordinary nature of the cases dictates that unorthodox means of investigation be utilized, but her objection is overruled.

Meanwhile, Eugene Tooms, a serial killer with unusual abilities, is interviewed by a parole board. Mulder, who helped incarcerate Tooms, testifies at the hearing. Those present at the review question Mulder's credibility when he claims that Tooms' murder spree spanned some 100 years, and that Tooms is capable of elongating his body - a genetic mutation also dictating that he ingest human livers.

Tooms is released from the sanitarium and placed under the care of Doctor Aaron Monte. Mulder is certain that Tooms will commit another murder the moment the opportunity presents itself. While Mulder keeps constant surveillance over Tooms, Scully researches the earlier murders hoping to prove that Tooms was responsible.

Scully calls on the services of Frank Briggs, a retired police detective who accumulated evidence surrounding the murders-a series of five killings that occur once every 30 years. He notes that in 1933, a human liver was discovered near a then-under-construction chemical plant, but the body of the victim was never recovered. Briggs believes that Tooms hid the body because something about the corpse could tie him to the killing.

Scully and Briggs venture to the chemical plant and recover a body entombed deep inside the cement foundation. Using sophisticated computer enhancement, a bite mark on the victim's body is matched to Tooms' dental records.

Tooms charges that Mulder beat him up. Mulder is arrested and interrogated. He vehemently denies the false allegations but cannot disprove them. Scully backs him up by claiming she was with him when the beating allegedly transpired. Mulder is released, but warned to stay far away from Tooms.

Mulder and Scully discover the body of Dr. Monte. Realizing that Tooms will try to hibernate for another 30 years, Mulder and Scully rush to the location where he had previously "nested." In place of the dilapidated building stands a shopping mall.

Mulder guesses that an escalator shaft is the likeliest place Tooms would have made the new nest. He climbs inside the darkened shaft, a flashlight illuminating his path. Tooms suddenly springs out of the darkness and attacks Mulder. With Scully's help, the agent struggles free. Tooms becomes entangled in the escalator's gears and is crushed to death.