When Nibbler has a 5th birthday party, Bender becomes annoyed that he 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 Nibbler down the toilet. Leela is distraught at the loss of her pet and angered by Bender's inability to 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 is forced to experience as well.
After a night of experiencing Leela's feelings, Bender misses Nibbler so much that he cannot stand it. When Fry tells him that alligators can supposedly live after being flushed, Bender flushes himself down the toilet in pursuit of Nibbler. Fry and Leela enter the sewers and quickly manage to find Bender. Unfortunately, they also find a crowd of mutants who live in the sewers. (Among them, Leela's mother, Turanga Munda, as by this point David X. Cohen and Matt Groening had planned her future) 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 sacrifice in the form of a virgin. Although not one herself (owing to the events of an earlier episode), Leela is the closest thing to a virgin there and is therefore forced to be the sacrifice. At the sacrifice, Nibbler emerges from a pipe - but so does a large, reptilian monster, 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.
The Professor removes Bender's empathy chip. He says in amazement that the chip was actually turned off, so that Bender was actually sympathizing for Leela by himself. He then corrects himself, saying that it was actually at triple capacity ("And I still barely felt anything!" Bender claims.) Bender still retains his 'in-your-face' attitude; meanwhile, Leela learns that adopting Bender's attitude is better than being nice.