The boys play "Texans vs. Mexicans" at a sleepover at Cartman's house, a game in which the "Mexicans" (led at first by Butters, and then Kyle due to the others seeing the former as a poor leader) attempt to elude the "border patrol" (led by Cartman) and cross the border into Texas (Cartman's backyard). All of the boys successfully do so except for Butters, who gets lost. His absence goes unnoticed by the rest of the group. Cartman, believing that the game has been lost, becomes dejected. It was only while waking up from a nightmare that Cartman realizes that Butters never returned and thus the game isn't over. He wakes the rest of the group. Concerned, they begin searching for their missing friend while Cartman and his crew resume protecting the backyard.
As it turned out, while attempting to reach Cartman's house at night during a snowstorm, Butters is struck by a vehicle. The couple in the vehicle, thinking Butters to be a Mexican immigrant named "Mantequilla" (which is Spanish for "Butter"), take him to their home and employ him as a stereotypical Mexican servant, giving him menial jobs to perform such as cleaning windows and washing dishes. However, the couple eventually abandons Butters at an El Pollo Loco restaurant, believing that Butters needs to be with his "own kind" to be happy. Inside, Butters regales the staff with tales of his imaginary exploits. The staff disregards him until he gives his name as "Mantequilla", which they recognize from some flyers that the boys have posted while searching. Believing him to be someone famous, they heed his words and start to question the value of their new lives in the United States. They misinterpret Butters' desire to "cross the border" and rejoin his friends as a genuine desire to return to Mexico, and a mass exodus of Mexican immigrants soon begins. As more and more Mexicans leave, chores pile up and the Americans can't keep up with their new found hard labor tasks creating a large gap in the economic balance and forcing the U.S. economy to fall severely.
The United States Border Patrol, now tasked with preventing Mexicans from leaving the United States, is quickly overmatched and begins recruiting volunteers. Cartman eagerly signs up and quickly displays an aptitude for stopping Mexicans, but the exodus continues. Butters, as "Mantequilla", is hailed as a hero in Mexico and is credited with instilling a sense of national pride in the Mexican people. However, he soon becomes homesick and attempts to return to the United States. While attempting to cross into the United States, Butters is spotted by border patrol agents, who are overjoyed in their belief a Mexican wants to enter the United States and make no attempt to stop him. But recognizing Butters, an enraged Cartman tries to prevent him from crossing the border. But with the Border Patrol's help, Butters successfully eludes Cartman and re-enters the United States, winning the game as well as returning home. Cartman once again becomes despondent as the gang is glad Butters is back, but though they still think he would not make a good leader. Butters proves the others wrong by raising his arms as the voices of the Mexican people chant in response.