"The Poor Kid" is the fourteenth and final episode of Season Fifteen and the 223rd overall episode of South Park. It aired on November 16, 2011.
With their parents in jail, Kenny McCormick, his brother, and sister find themselves in the foster care system. Eric realizes he is the poorest kid in school in Kenny's absence, and so Mysterion must fin a way to save them all!
Kenny's parents are arrested for having a meth lab at their home, an event documented on the reality show White Trash In Trouble. As a result, Kenny and his two siblings are placed with the Weatherheads, a militantly agnostic foster family living in Greeley that forbid their numerous foster children from expressing any notions of certainty. Their agnosticism manifests itself in a number of peculiar ways, such as their edict that the children can only drink "agnostic beverages" like Dr. Pep-er, because no one can be certain as to what flavor it is, and hypothesizing that God could be a "giant reptilian bird" in charge of everything. Because of his younger sister Karen McCormick's sadness and fear over their new living situation, Kenny attempts to protect and comfort her by adopting his Mysterion persona, whom sees as her guardian angel.
While searching for the "new poorest kid" with Butters, Cartman learns that he is now the poorest kid attending South Park Elementary. Cartman complains to his mother, but she says that she is already working two jobs and cannot do more because of the economy. Wishing he were in a foster home like Kenny, Cartman frames her for running a meth lab. Despite her protests of not having done drugs in a long time, she is arrested, which is again documented by White Trash in Trouble. Despite anticipating being sent to an idyllic setting like Hawaii, Cartman is placed with the Weatherheads, and attends the same school in Greeley as Kenny, where he is elated to discover that the poorest student there is not him, but a boy named Jacob Hallery, whom Cartman takes delight in ridiculing because he's not the poor kid in school. This causes him to be sent to the principal's office for insulting him.
Jessica Pinkerton, who was harassing Karen, gets beaten up by Mysterion. Karen and the other foster kids report what they saw to the Weatherheads, calling Mysterion an angel. Having expressed certain belief in the divine, Mr. Weatherhead waterboards them with Dr. Pepper. In response to the Weatherheads' cruelty, Cartman reports them to Child Protective Services, while Kenny, as Mysterion, plants beer in their refrigerator, on which they get drunk, thus getting arrested for themselves being unfit guardians, as documented on White Trash In Trouble. Cartman is also arrested for filing a false police report, and his and Kenny's caseworker, Mr. Adams, urges the police to return all the foster children to their parents, as the foster care system has been embarrassed. Kenny and Cartman return home, but on their first day back at South Park Elementary, a giant reptilian bird, rips open the school's roof and eats Kenny, prompting Cartman to burst into tears that he is the poorest kid in school once again, makes a "yo mama so poor" joke about his own mother and crying about how poor he is.