Writer: Chris Carter
Director: Daniel Sackheim
Mulder later confesses to Scully that he is considering leaving the bureau and pursuing his interest in the paranormal on his own. But Scully, handling her reassignment more professionally, insists on doing the autopsy and discovers a parasitic worm in the dead man's body. When a sewage worker is attacked by something in the same sewers, Mulder interviews the victim in hospital, he has a large unidentified bite mark on his back from the attack.
Mulder receives a phone call, an unknown male voice tells him he has a friend at the FBI. Mulder assuming Scully has been talking about him leaving, asks her not to start any crusades for him, but Scully denies any knowledge on the phone call. When the sewage worker is later found dead in his apartment, Mulder realises that the worm is merely the lava stage of a larger creature, that the perpetrator in the murders is a mutant flukeworm. Scully's investigation reveals the new species arrived on a ship used to dispose of material from the Chernobyl accident. Scully concludes that nature didn't make it, mankind did.
Gabrielle Rose returns as Dr. Zenzola: she previously appeared in the episode ''Deep Throat''.
During Scully's autopsy of the victim, she says the case number is DP112148 (Chris Carter's wife's birthday) preceded by her initials. She says the victim is John Doe 101356, Chris Carter's birthday.
This episode marks the first appearance of X, Deep Throat's successor as Mulder's confidential source who phones him to say that he has 'a friend in the FBI'. Although the actor on the phone isn't Steven Williams, the voice was dubbed over with Williams' after he was cast in the part for ''Sleepless''.
X: Mister Mulder, I will make this brief. Success in your current assignment is imperative.
Mulder: Who am I speaking to?
X: Are you hearing me, Mister Mulder?
Mulder: Yes. Why is it imperative?
X: Reinstatement of the X-Files must be undeniable.
Scully: Is this seat taken?
Mulder: No, but I should warn you I'm experiencing violent impulses.
Scully: Well, I'm armed. So I'll take my chances. I hear you really endeared yourself to Assistant Director Skinner today.
Mulder: Sometimes it just gets hard to smile through it when they ask you to bend down and grab your ankles, you know?