When Bender flushes Nibbler down the toilet, Leela has Bender implanted with a chip that lets him feel what she's feeling. When the feeling of loss become too much, Bender sends himself down the toilet on a rescue mission. Fry and Leela must now journey down for their lost friends, encountering sewer mutants in the process.
- Absurdly Spacious Sewer: This episode shows the first actual sewers that arent flooded ruins, and they consist of tunnels as wide as streams and with several meters of air-filled overhead, way more than a sewer would need. Later subverted when Fry says their only way out is through a sewer pipe a few centimeters around, although it widens—after a mile.
- Ambiguously Gay: Feeling Leela's jealousy for Amy, Bender yells at Fry that the reason he "gets all the boys" is because he dresses like a tramp. Fry's response?Fry: They're just responding to my personality!
- Big Shadow, Little Creature: The shadow of El Chupanibre turns out to be little Nibbler. Subverted when he is followed by the real Chupanibre.
- Chupacabra: Slightly respelled as the sewer-dwelling El Chupanibre, although it doesnt share much with the original creature besides its name and disbelief in its existence, being a hulking reptilian predator instead of a Grey-like vampire.
- Dramatic Irony: Only really comes out on rewatchings, but hearing Leela of all people claim that theres no such thing as mutants definitely fits.
- Early-Bird Cameo: Leela's parents can be seen among the other mutants in crowd scenes.
- Early Installment Character-Design Difference: In her first appearance, Turanga Munda is drawn with human-looking arms instead of the tentacles she's later depicted as having.
- Emotion Chip: Farnsworth installs an empathy chip that allows (or rather, forces) Bender to feel what Leela feels.
- Made Myself Sad: Inevitably, Bender insulting Leela results in this since he shares her emotions.
- Magic Feather: At the end of the episode, the Professor goes to remove the empathy chip and is surprised to discover that it had shorted out a while ago. The emotions Bender felt were genuine! ...Then he looks again and realizes he was wrong. The chip is actually working at triple capacity.Bender: And I still barely felt anything! Good night, losers!
- Matchlight Danger Revelation: Parodied. Fry lights a match inside the sewer and screams... because the match burned his finger.
- Moral Myopia: Leela really isn't much better than Bender here; she is highly indifferent when he is mutilated by the can opener.
- The Power of Apathy: When Bender has to save Nibbler from a monster, he can't because Leela is too emotionally distraught. So Bender has to teach her to not care about Nibbler... for the sake of saving Nibbler.Bender: Listen to me, Leela. I'm an expert at not caring. The secret is to stop giving a rat's ass about anyone else and start thinking of the things that you want, that you deserve, that the world owes you.
Leela: Well, I could use a new tank top.
Bender: Bigger! Bigger!
Leela: A fashionable tank top, and fashionable boots... encrusted with jewels.
Bender: Don't stop now! You need pants to go with that outfit!
Leela: Yeah! And I could afford it all if I didn't have to feed that stupid Nibbler!
Bender: Bender is back! I'll save you, Nibbler!
- Sewer Gator: Averted. The Sewer Mutants don't have alligators. They have crocodiles.
- Shout-Out: Fry notices that the mutants worship an atomic bomb, but the mutants admit that it's "more of a Christmas and Easter thing".
- Ship Tease: When Fry asks Leela if she loves him too, she says as a friend. He's disappointed.
- Stealth Pun: After Bender threatens Nibbler, Nibbler bites his shiny metal ass.
- Take That!: The mutant's library has only books that have been flushed down the toilet, which Bender points out consist of "nothing but crumpled porn and Ayn Rand."
- Toilet Humour: The Professor speculates that part of the reason the mutants mutated was "good old American faeces."
- Virgin Sacrifice: Leela offers herself as one to lure out El Chupanibre, but the mutants have seen Zapp Brannigan's website. They go along with it regardless.
- You Killed My Father: Parodied by Bender.Bender: Stupid can opener! You killed my father and now you've come back for me!