I Second that Emotion

After flushing Nibbler down the toilet, Bender is forced to install an empathy chip. He ends up being upset about Nibbler, and starts an expedition to save him. This leads the crew through the sewers to the that is El Chupanibre.

Act I: "How could you flush Nibbler down the toilet!"
After Nibbler breaks a fang after literally biting Bender's shiny metal ass, they have to take him to the vet to get a new tooth. He receives a replacement tooth from the vet. The vet speculates that Nibbler may be 5 years old, so the crew throws him a birthday. Bender, jealous of the attention Nibbler is getting, bakes an amazing cake stating Bender is great, which Nibbler eats before anyone can see it. In anger, Bender flushes Nibbler down the toilet.

Act II: "There's mutants down there!"
Leela is inconsolable. Everyone sees that Bender is completely incapable of understanding empathy, so the Professor equips him with an Empathy Chip that "allows" (i.e. forces) Bender to feel all of Leela's emotions as she feels them--and not just sadness, but happiness, anger, jealousy and fear. Unable to bear the pain (and fed up with feeling the emotions), Bender flushes himself down the toilet to go hunt for Nibbler. In spite of the urban legends about a race of hideous mutants who live there, Fry and Leela enter the sewers to search for Bender. After finding him they realise they are lost, and while trying to find a way out they run into the dreaded sewer mutants.

Act III: "Aren't you supposed to be eating our brains?"
The Mutants turn out to be harmless and take them to their city, telling the legend of a dreaded monster known as El Chupanibre. Leela begins to wonder if Nibbler is the monster, so the mutants, not wanting to take any chances, use her as bait to capture the monster. Nibbler soon appears, but the mutants confirm that he is not the monster--in fact the real El Chupanibre, a huge, vicious, drooling monster, is right behind him. Bender attempts to fight it off, but Leela's fear and sense of concern for Nibbler's safety hinders him. In order to defeat the monster, Bender convinces Leela to act heartless and self-centered, giving him the capacity to knock him into the giant toilet and flush him into the sub-sewer.

Back at the Planet Express building, the Professor removes the empathy chip and sees that it was actually damaged, apparently indicating Bender's emotions were real, only to correct himself when he sees that it was actually running at three times normal capacity. Bender has learned nothing and leaves, but Leela claims that she herself has learned from him and leaves the room with a curt "So long, jerkwads!"

Reception
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Trivia

 * Nibbler eats a big can of Kibbles 'N' Snouts.
 * Bender whistles the Harlem Globetrotters' theme song, "Sweet Georgia Brown," three times in this episode. He may be whistling this song in particular because he says in the later episode "Time Keeps on Slippin'" that he dreams of being a Harlem Globetrotter.
 * The crew sings an edited version of "" at Nibbler's party. This is because the actual song is copyrighted and the studio would have had to pay to use it.
 * This is the first appearance of Leela's parents (background)
 * Turanga Morris's mouth is normal rather than sideways.
 * Turanga Munda has arms instead of tentacles in this appearance.
 * Bender says "bite my shiny metal ass" in several episodes, usually as a comeback. This is the first episode in which someone performs such an act - Nibbler.
 * The catchphrase itself is not used at any point during this episode.
 * The vet says, "if [Nibbler's] anything like the common tree, the rings might indicate his age"..

Allusions

 * El Chupanibre is a reference to.
 * The scene where they offer a sacrifice to El Chupanibre is a reference to .
 * During the scene in the vets office a  monster (a ) is seen.
 *  is seen amongst a pile of sewage and garbage.

Goofs

 * The bathroom that features prominently in this episode is through the door that people usually enter from. So either the bathroom is in the wrong place, or people must enter the TV area by passing through the bathroom.
 * Bender comments that lifting the seat is an annoyance, implying that he must know how men use toilet. Yet when he flushes himself out, he comments "what do those humans design this for anyway?"
 * Bender may have heard about the seat from somebody else.
 * It is said that mutants are a myth in this episode, yet in "I, Roommate" Bender says, "or we could live underground with the mutants, a little fire'll show 'em who's boss."
 * Amy and Zoidberg thoroughly believed in mutants, so it's possible some people believe in mutants, and some don't.
 * This could have been a reference to the many movies and books where there are underground mutants. This would also explain him referencing fire, because mutants outside the series are usually afraid of and blinded by light and fire.
 * When Fry is released from the trap, there is an off-screen splash into the mutant pond, yet there is no mention of any danger of mutation.
 * It was probably another rat.
 * Bender is shown having trouble re-assembling himself in this episode, yet both in prior episodes and since, he's had no trouble at all removing and replacing parts of himself.
 * He may have had trouble due to the chip in his head.
 * Leela pushes Fry down in to the sewers. He lands in sewer water and does not mutate. This happens later on, where he is freed from the rope - he falls into the sewer water again. This conflicts with the events of "Leela's Homeworld" and "The Mutants Are Revolting" - once a human enters the sewer water, it should mutate.
 * It's possible that this only apples to Lake Mutagenic as opposed to all sewer water everywhere.
 * When Fry, Leela and Bender are shown round the sewers, there is a sign above a dry cleaners which says "DRY", and underneath "CLEANERS". In the next shot, it just says "CLEANERS".

Characters

 * Amy
 * Debut: Armando
 * Bender
 * Calculon
 * Debut: Dave Spiegel and Fluffers
 * Debut: Dwayne
 * Debut: El Chupanibre
 * Fry
 * Hermes
 * Leela
 * Debut: Leg Mutant
 * Nibbler
 * Prof. Farnsworth
 * Debut: Raoul Inglis
 * Debut: Rover and Pepper
 * Debut: Turanga Morris and Munda (background)
 * Debut: Vet Jeffrey Grant
 * Debut: Vyolet
 * Zapp (mentioned)
 * Zoidberg

Episode Credits

 * Writer
 * Patric M. Verrone
 * Director
 * Mark Ervin
 * Voice Actors
 * Billy West
 * Katey Sagal
 * John DiMaggio
 * Phil LaMarr
 * David Herman
 * Lauren Tom
 * DVD Commentary
 * Matt Groening
 * David X. Cohen
 * Rich Moore
 * Patric M. Verrone
 * John DiMaggio