Kyle's parents are taking him and 3 of his friends to Casa Bonita, Colorado's version of a Mexican Disneyland. When Kyle chooses to invite Butters instead of Cartman, the fat boy plots to do away with Butters and keep Casa Bonita to himself.
This episode starts off with Stan Marsh wondering where Kyle is, followed by Eric Cartman saying he hopes he caught a disease and died, with Stan and Kenny reprimanding him. Kyle arrives and announces to the group that as a birthday treat his mother is taking him and three of his friends to Casa Bonita, a restaurant that Cartman dreams about frequently. Cartman is ecstatic and begins singing the "Casa Bonita Song" until Kyle disappoints him by revealing that he has decided to invite Butters to his birthday party rather than him. Kyle explains that Cartman constantly mocks him for being a Jew (this results in a montage of all the times that Cartman has made fun of him), and that Butters is a better friend to him than Cartman. After having a nightmare in which he is pulled out of Casa Bonita, Cartman sets out on a campaign to prove that he is nice; however, kindness is such a foreign concept to Cartman that his attempts are laughably unsuccessful, and Butters is still invited. Cartman's next step is to talk to Jimmy asking him to teach him to be nice. Cartman can't grasp any of what Jimmy tells him, and when Kyle passes by, he proceeds to beat up Jimmy, pretending to be defending Kyle's mother from Jimmy saying he had had sex with her. This prompts Kyle to question, "You really think beating up handicapped kids is nice?". After lunch, Cartman talks to Kyle, saying that he was sorry for all the past abuse he had inflicted upon him and saying that he hoped they could still be friends despite the predicament. Kyle, touched and knowing how much Cartman wanted to go to Casa Bonita, says that, if Butters is for some reason unable to go, Cartman can take his place. Cartman takes advantage of this and starts planning on getting rid of Butters.
Cartman tricks Butters into thinking that a meteor about the size of Wyoming is about to hit Earth, and arranges for him to go into hiding to escape the disaster; and, with Butters gone, Cartman figures he's set to go to the restaurant with Kyle. He hides Butters into Jimbo's bomb shelter filled with food and water. On Saturday night, everyone is ready to go and Butters has not showed up yet. Cartman arrives to give Kyle a birthday present, then minutes later, Stephen and Linda show up tearfully asking if they have seen Butters. Kyle says that it "doesn't feel right" to go and enjoy himself now that Butters is missing. Kyle decides to postpone the party for next week. The policemen state that they will be checking bomb shelters as well, which is bad news to Cartman. Cartman says that a meteor the size of Wyoming has hit and that the world is filled with radioactive cannibals. Cartman makes Butters wear a cardboard box over his head so he doesn't see that there hasn't been a meteor, saying that the destruction is too terrible for Butters to see. Cartman brings Butters to the abandoned gas station because it looks horrible enough. Cartman then tells Butters that a radioactive cannibal just came and Cartman managed to kill it, but that it bit him so Cartman will turn into one as well. Cartman puts Butters into an abandoned refrigerator; seconds after Cartman leaves, a garbage truck comes and dumps Butters into the city dump. Butters thinks the dump is the remainders of Earth and after finding a dog, they build a "new civilization". While he is doing so, he finds a junkyard woman and thinks she is another survivor, and asks her to help rebuild the civilization. When Butters explains Cartman's story, the woman explains that none of it actually happened: there was no meteor, no-one was killed, and the town has been looking for Butters for over a week. Butters, upon realizing that Cartman tricked him, asks the woman for her phone.
During the drive to Casa Bonita, Kyle is slightly suspicious as to whether or not Cartman is really nice when Stan suggests he (Kyle) decides what to do first as it's his birthday, and Cartman says, "What? Fuck Kyle!" and automatically tries to correct his damage by leading everyone into singing "Happy Birthday." When they arrive, Kyle's mother gets a call telling her that Butters was found, and about Cartman's involvement, confirming Kyle's doubts. She and the other boys confront Cartman, notifying him that the police are on their way. Knowing that his entire plan is close to ruin, Cartman escapes and goes through a mad dash through the restaurant to sample every attraction and food in the minute he has left. He is eventually trapped and cornered by the police, the Broflovskis and the boys, and he jumps off the waterfall. They catch up with him floating on his back in the water below. When a police officer explains all the trouble that Cartman's caused and that he's going to be sent to Juvenile Hall for a week, he asks Eric if it had been worth it. Cartman responds dreamily, "....totally", before the credits roll.