An old Academy buddy of Scully's asks her and Mulder to consult on a case he is having trouble with - but when Mulder suggests that the seemingly impossible murders are being carried out by a hundred-year old cannibalistic mutant who attacks on a regular 30-year cycle, their help is no longer welcome. With the help of the cop who investigated the killer in the 1960s, they are able to apprehend the killer.
- Air-Vent Passageway: Justified.
- Anthropomorphic Personification: Frank Briggs speculates that Tooms is some kind of personification of all of the worst, most evil impulses of humanity given human form.
- Body Horror: Tooms's transformation.
- By the Lights of Their Eyes: Tooms's eyes turn yellowish when he contemplate his future prey.
- Creepy Long Fingers: Tooms.
- Creepy Monotone: Tooms's voice.
- Comfort Food: Mulder and Scully are seen carrying a snack bar and a pack of sunflower seeds after Colton's team releases Tooms due to him "passing" the polygraph test.
- Damsel in Distress: Scully finds herself seconds away from experiencing her liver being torn away. However, she puts up a brave fight, which buys her enough time to let Mulder intervene and save the day.
- Fingertip Drug Analysis: Mulder sticks his fingers into Tooms's nest without wearing a rubber glove and ends up with bile on his fingertips. As a result:
Mulder: Is there any way I can get it off my fingers quickly without betraying my cool exterior?
- Foreshadowing: Mulder's reaching out to touch Scully's pendant during a conversation serves as a directorial NOTICE THIS! for later in the episode when the pendant becomes Tooms' next trophy.
- For the Evulz: Tooms needs to get human livers for hibernation, but he also obviously kills for the fun and because he enjoys it. He deliberately chooses places that are a challenge to get into as if it were some kind of sport. Moreover, he takes trophies from his victims, sometimes in advance.
- The Gadfly: Played with; Mulder is mocked for his genuine beliefs in the supernatural and paranormal, but admits to a tendency to play it up around people who are particularly dickish about it just to mess with them, as with when he suggests to Tom Colton with a straight face that an alien race called 'Reticulans' are responsible for the killings due to a bustling liver trade.
Scully: You knew they wouldn't believe you; why did you push it?
Mulder: Maybe I thought you had the right guy. And maybe I run into so many people who are hostile just because they can't open their minds to the possibilities that sometimes the need to mess with their heads outweighs the millstone of humiliation.
- Gunman with Three Names: Eugene Victor Tooms.
- Hey, It's That Guy!: Donal Logue
- Horror Hunger: Tooms is a genetic mutant that feeds on human livers. Five of these will provide him sustenance for 30 years. However, he can survive and function without them.
- Hungry Menace: People, beware, Tooms needs five human livers.
- I'm a Humanitarian: Tooms, a human genetic mutant, eats humans.
- Jerkass: Agent Tom Colton. He calls in Scully (and Mulder) for help, but quickly compromises the investigation when it turns out to really be an X-File, and proves to really only be interested in advancing his career.
- Contrasted with Mulder, who may behave like a jerkass towards Scully - definitely towards Colton - but proves more open and supportive of Scully's actions. When Scully turns out to be right about staking out Tooms' last murder scene, Mulder immediately admits that she made the right call.
- Jurisdiction Friction: Interestingly, it's FBI versus FBI. More specifically, Violent Crimes Section versus The X-Files Division.
- Lie Detector: FBI uses a polygraph to question Tooms. He fails Mulder's questions about his presence at one old crime scene and being over 100 years old that he didn't anticipate. However, Colton releases him anyway.
- Locked Room Mystery
- Monster of the Week: Established the pattern very successfully.
- Mutants: Tooms is a genetic mutant capable of elongating his body.
- Oh Crap: When Mulder recognises Scully's pendant among Tooms' trophies.
- Older than They Look: Tooms looks like he's in his late twenties, but his first murder dates back to 1903.
- Picky People Eater: Tooms only eats his victims' livers.
- The Profiler: Tom Colton and Dana Scully. She absolutely nailed the profile.
- Played with for Mulder, who had worked as a Profiler before finding the X-Files. He's convinced the serial killer wouldn't return to a previous crime scene as Scully predicts. Scully turns out to be right as they catch Tooms doing exactly that. Mulder immediately admits he was wrong.
- Regularly Scheduled Evil: Tooms comes out of hibernation every thirty years.
- Serial Killer: Tooms kills five victims in each cycle. His victims vary in age, race, gender, with no connections to each other. The pattern was the lack of point of entry and extracted human livers by bare hands.
- Sizeshifter: Tooms is capable of contorting and elongating his body.
- Slow Motion: Tooms's second on-screen attack.
- The Stakeout: Mulder and Scully set up one right outside Tooms' residence, only for their plan to be shredded by Tom Colton.
- Supernatural Gold Eyes: When Tooms attacks, his eyes glow bright yellowish-green. The color of bile, appropriately.
- That One Case: Frank Briggs was the investigating officer of the murders in the 1930s and the case kept haunting him.
- They Look Just Like Everyone Else: Tooms is able to pass for a sanitation animal control worker.
- Trademark Favorite Food: Mulder's sunflower seeds.
- Uncleanliness Is Next to Ungodliness: Tooms's apartment is a hell-hole of a place and his nest is made out of rags and newspapers with bile all over it.