Some info and a short description of this South Park episode on South Park Zone:
South Park - All About Mormons, is the 12th episode of the seventh season of the animated television series South Park. It was originally broadcast on Comedy Central in the United States on November 19, 2003.
This south park episode revolves around the religion and culture of Mormons, as a Mormon family moves to the town of South Park, and influences the beliefs of the family of character Stan Marsh. The story of Joseph Smith's founding of Mormonism and the writing of the Book of Mormon is told through a number of comedic 19th century flashbacks, with a musical narration.
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(The complete plot for this South Park Episode)
This South Park episode starts with a new family, the Harrisons, move into South Park, and their son Gary, stereotypically depicted as unusually perfect, achieving high grades, being state champion in sports, being polite and so on, invokes the wrath of the other boys. Stan has the job (from the other kids in school) to beat up Gary, but Gary's sheer politeness leads Stan to discover himself walking away with an invitation to dinner that night. After dinner, the five-child, two-parent family has "Family Home Evening" where they play games, do performance art and read from the Book of Mormon. Stan is intrigued and confused by all this, and asks his parents about the Mormon family's beliefs. His father concludes that they must be religious fanatics attempting to brainwash Stan, and heads over to confront Mr. Harrison and beat him up. Instead, he too finds himself quelled by the family's perfection and politeness, and in the end, actually decides to convert to Mormonism himself. The next day, Kenny, Cartman and Kyle cruelly mock Stan for hanging around with Gary and his family, accusing Stan of going on a date with Gary. When the Harrisons and Gary show up, the three children walk off lying about going to "put in some volunteer work at the homeless shelter".Throughout the episode, characters ask questions about Mormonism, and the story then breaks off to a sub-story about Joseph Smith, Jr. and the founding of the religion. For satirical purposes, the show deviates from the original accounts of Mormonism's founding by adding extra to stories originally left vague (e.g. the precise location where Martin Harris lost the transcript of the Book of Lehi given to him by Joseph Smith); furthermore, during the narration, an upbeat tune plays in the background, with a choral "Dum, dum, dum, dum, dum" following the lyrical lines of the song. When skeptic Lucy Harris appears in the sub-story, the chorus changes to "Smart, smart, smart, smart, smart," and it becomes clear that the voices are actually singing "Dumb, dumb, dumb" after the specifics of Smith's story. The show asserts flaws in the religion's founding, which especially concern Stan (for example, that Joseph Smith offered no proof to the general public of finding the Golden plates, and that he claimed to have translated from a slightly different plate after the first translation was lost while in the possession of Martin Harris). Stan ends up shouting at the Mormons that they're ridiculous for believing in it without proof; they smile patiently and explain that it's a matter of faith, while Stan argues that it should be a matter of empirical evidence. He further lashes out at them for acting unusually nice all the time, claiming it blindsides stupid people like his father into believing in Mormonism (to which Randy Marsh responds with a determined "Yeah!").
Stan's anger doesn't much upset anyone in the Mormon family other than Gary, who confronts Stan and the other boys the next day, pointing out that he believes his religion does not need to be factually true, because it still supports good family values and helping the poor. Gary condemns their bigotry and ignorance in language that is normal for the main characters but extremely surprising and powerful coming from Gary. He walks away, leaving the boys in utter shock. The episode ends as Cartman, with a new-found respect for Gary, says "Damn, that kid is cool, huh?".