I Second that Emotion

Act I: "How could you flush Nibbler down the toilet!"
After Nibbler breaks a fang biting Bender's shiny metal ass, the vet speculates that Nibbler may be 5 years old, so they throw him a birthday party. Bender, jealous of the attention Nibbler is getting, bakes an amazing cake, 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, and everyone sees that Bender is completely incapable of understanding empathy, so the Professor equips him with an Empathy Chip, "allowing" (i.e. forcing) Bender to feel all of Leela's emotions as she feels them--and not just sadness, but hapiness, 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 - that is, until they come to believe that Nibbler is the dreaded El Chupanibre. The mutants capture them and take them to their city, where Leela is used as bait for El Chupanibre (who all believe to be Nibbler). Nibbler appears, but right behind him is the real El Chupanibre, a huge, vicious, drooling monster. Bender attempts to fight it off, but has his arms torn off. Things look bad, until Bender's arms distract the monster long enough for Bender to knock him into the giant toilet and flush him into the sub-sewer.

Trivia

 * Nibbler eats a big can of Kibbles 'N' Snouts.

Quotes

 * Calculon: Give it to me straight, doctor. Don't sugar-coat it. DoctorBot: All right. Your entire family died when a plane piloted by your fiance crashed into your uninsured home, and you have inoperable cancer. Bender: [laughs hysterically] Bet you didn't expect that one, Calculon!

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 humans use it for anyway?"

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

Episode Credits

 * Writer
 * Patric M. Verrone
 * Director
 * Mark Ervin
 * Voice Actors
 * Billy West
 * Katey Sagal
 * John DiMaggio
 * Phil LaMarr
 * David Herman
 * Lauren Tom