"I Second That Emotion" is the first episode in season two of Futurama. It originally aired on the Fox network in the United States on November 21, 1999. The episode was written by Patric Verrone and directed by Mark Ervin. The episode introduces the recurring sewer mutants, a society of humans who have been mutated by years of exposure to pollution and radioactive waste poured into the sewers from New New York.

When Nibbler chips his fang, he is taken to a vet, and during the time, is found out to be five years old. Nibbler has a birthday party, and Bender becomes annoyed that Nibbler is getting more attention than him. Having reluctantly made a birthday cake for Nibbler, Bender is aghast to see Nibbler gobble it all up before everybody else can praise Bender for the cake. In extreme annoyance, Bender flushes the Nibbler down the toilet. Leela is distraught at the loss of her pet and wishes Bender could understand the emotions of others. Professor Farnsworth proposes a solution: installing an empathy chip in Bender's head that will cause him to feel other people's emotions. After forcibly installing the chip, the chip is tuned to pick up Leela's emotions, so that whatever feelings Leela experiences, Bender automatically experiences them as well.

After a night of experiencing a multitude of Leela's feelings, Bender misses Nibbler so much that he cannot stand it any longer. When Fry tells him that alligators can supposedly live after being flushed, Bender flushes parts of himself down the toilet in pursuit of Nibbler. Fry and Leela enter the sewers and quickly manage to find Bender and a crowd of mutants who live in the sewers. The mutants introduce them to their subterranean civilization. They also reveal that a monster called El Chupanibre has been terrorizing them.

Leela, thinking that Nibbler is the monster, is happy until she is informed that the only way to lure the monster out is to offer a "snackrifice" in the form of a virgin. Although not one herself (because of her run-in with Zapp Brannigan), Leela is chosen anyway to be the sacrifice. At the sacrifice, Nibbler emerges from a pipe: however, the mutants point out Nibbler is not the beast they fear, but the large, reptilian monster behind him, who is the real El Chupanibre. Fry gets entangled in the trap that was meant for El Chupanibre, leaving only Bender to fight the beast; however, Leela is so scared, for herself and the others, that Bender is immobilized by her emotions. He and Fry teach Leela to stop caring about other people and care only about herself, and as a result, Bender is able to fight off the beast, whom he eventually flushes down a giant toilet into the sub-sewers.

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