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 Dedicated Worker: One of the most prominent cases where Nibbler being The Mole makes his behaviour in this episode border on Too Dumb to Live. He ceaselessly tempts fate by infuriating Bender with his destructive pet persona, and he makes no attempts to defend himself from getting flushed down the toilet or run from the El Chupanibre. He supposedly even keeps up the act when alone in the sewer for days.
- Absurdly-Spacious Sewer: This episode shows the first actual sewers that aren't 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.
- Air Quotes: The three-armed mutant does this with all three hands while explaining the plan to use Leela as a "virgin" to lure El Chupanibre into a trap.
- Ambiguously Bi: Feeling Leela's jealousy for Amy, Bender yells at Fry that the reason he "gets all the guys" is because he dresses like a tramp. Fry's response?Fry: They're just responding to my personality!
- As Long as It Sounds Foreign: "El Chupanibre" is clearly meant to be a variation of the Spanish name for the famed monster El Chupacabra, "The Goatsucker"... But the word "nibre" doesn't mean anything in Spanish, so El Chupanibre would translate to "The Nibresucker".
- After Leela tells him to bake a cake Bender is seen sprinkling rat poison. You think he's planning to poison everyone but he was genuinely using it to get rid of rats.
- At the end, the professor mentions the empathy chip burned out, suggesting that Bender's empathy for Nibbler was of his own free will and not a result of the chip. He then takes it back a second later and reveals the empathy chip was actually running at triple capacity; Bender then confirms he barely felt even that.
- When the Professor messes with the emotion chip to try and tune Bender with Leela's feelings, Bender says that he's worried about being not as smart as Leela, but relieved that he's cuter than her. This initially seems to be Amy's feelings, but then Fry whispers "Thanks for covering", implying that he was the one thinking that.
- Big "NO!": Leela shouts it after catching Bender flushing Nibbler down the toilet.
- Big Shadow, Little Creature: The shadow of El Chupanibre turns out to be little Nibbler. Subverted when he is followed by the real Chupanibre.
- By the Lights of Their Eyes: Fry and Leela's eyes are shown in the dark when they enter the sewers.
- Chupacabra: Slightly respelled as the sewer-dwelling El Chupanibre, although it doesn’t 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.
- Compartment Shot: The shot of Bender snatching the rat poison from the cupboard.
- Disapproving Look: Fry gets this look from both Leela and Bender when he suggests that the only way out of the sewers is a narrow tube above their heads.
- Dramatic Irony: Only really comes out on rewatchings, but hearing Leela of all people claim that there’s no such thing as mutants definitely fits.
- The Dreaded: The mutants fear El Chupanibre.
- 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.
- Early-Installment Weirdness: Bender makes a perfectly edible cake in this episode, rather contrary to his later depiction as a Lethal Chef.
- Emotion Chip: Farnsworth installs an empathy chip that allows (or rather, forces) Bender to feel what Leela feels.
- Foreshadowing: In the crowd of mutants, we see two mutants who look just like Leela, hinting at her true origins. They would later be revealed to be her parents and she was actually a sewer mutant rather than an alien as she always believed.
- Happy Birthday to You!: At Nibbler's birthday party the crew sing a similar-but-different version of the traditional Happy Birthday song. This episode was made when Warner Music Group still claimed copyright on the lyrics to "Happy Birthday" but it also makes sense that the song may have changed by the year 3000. Fry, who would still know the 20th century version, is heard singing "And you smell like one too!" after everyone else finishes.
- A Head at Each End: At the veterinary, we see a dog with one head at each end.
- Heartbreak and Ice Cream: When Bender feels Leela's sorrow, he announces that he is going for a bucket of ice cream.
- Jumping Out of a Cake: This exchange:Leela: Just make a simple cake. And this time if someone's gonna jump out of it, make sure to put them in after you cook it.
- Just in Time: Bender saves Nibbler in the nick of time before El Chupanibre can swallow him.
- Last Grasp at Life: El Chupanibre's raised arm as he goes down the toilet.
- 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.
- No Sympathy: The crew gets so annoyed with Bender's lack of sympathy for anyone, that they install an emotion chip to his head, which makes him feel everything Leela (whose pet, Nibbler he flushed down the toilet) felt. Also applies to Leela herself, as she shows no concern for Bender getting cut by the can opener, getting angry at him for scolding Nibbler for the predicament, or Nibbler eating the cake Bender had made (which may not seem like much, but Bender had spent ages making it, and Nibbler ate it before anyone else had seen it), which partly led to the critter's flushing.
- Noodle Incident: Apparently, Bender once baked a cake with someone inside it.
- 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!
- Real After All: The crew initially believes that El Chupanibre is just Nibbler and that the mutants are just making up what they've heard about the beast on the fly. Upon finding Nibbler, Bender shows the mutants their "El Chupanibre". The mutants then reply that Nibbler's not El Chupanibre, the giant monster standing behind Bender is.
- Rewatch Bonus: During the massive crowd shot of all the mutants, viewers will be able to spot two one eyed mutants at the back, one with purple hair just like Leela's, setting the stage for the reveal that Leela isn't an alien, but a mutant human.
- Screams Like a Little Girl: Bender - under the influence of Leela's emotions - screams exactly like her when he sees El Chupanibre.
- Sewer Gator: Averted. The Sewer Mutants don't have alligators. They have crocodiles.
- 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."
- Tickertape Parade: One mutant suggests this after the victory over El Chupanibre. Bender tells him off.
- Titled After the Song: Named for a song by Smokey Robinson and the Miracles.
- Toilet Humour: The Professor speculates that part of the reason the mutants mutated was "good old American feces."
- Traitor Shot: Of Bender when he delivers the line "I'll make you a cake you'll never forget." Subverted in that, right after a brief Bait-and-Switch with rat poison, he then sets out to genuinely make a cake.
- Unspoken Plan Guarantee: The plan to trap El Chupanibre with a leash gets explained which doomed it to fail.
- 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.
- Who Needs Their Whole Body?: Bender can still fight with his arms removed. It helps that his detached arms can fight on their own.
- You Killed My Father: Parodied by Bender.Bender: Stupid can opener! You killed my father and now you've come back for me!