"Little Girl in the Big Ten" is the twentieth episode of Season 13.
In order to avoid a failing grade in gym, Lisa takes up gymnastics, which she uses to pose as a college student. Meanwhile, Bart is diagnosed with "Panda" virus and is forced to spend a week living in a plastic bubble.
Lisa's having trouble in gym and, to avoid failing, Mrs. Pommelhorse suggests that she enroll in a private gymnastics class that is taught by Coach Lugash. Lisa, at first, feels it is pointless, but after falling and hitting her head, she is visited by JFK in a dream who tells her that while academics are obviously very important, athletics are just as important, and Lisa chooses to enroll in the class. Lisa proves to be really good at gymnastics, which Lugash credits to her having a large head, which gives her perfect balance. After class, she meets two girls named Tina and Carrie, who happen to very similar to her. Lisa quickly realizes that Carrie and Tina are actually college students--they explain that because of their "small gymnast bodies," they're often mistaken for being much younger than they actually are. Lisa lies to them by saying that she is a college student, too, as she wants to earn their friendship and they give her a ride home in their car and invite her to one of Robert Pinsky's poetry readings.
Meanwhile, Bart is at Krusty Burger with his grandpa. He reaches in and pulls a Krusty-saurus toy out of his meal only for a mosquito from China to fly out the bottom of it and bite him and he becomes infected with the Panda Virus. To prevent it from being caught by any other people, Dr. Hibbert places Bart inside a giant plastic bubble. Bart has trouble adapting to life inside the bubble due to problems like not being able to eat and take a bath, although Homer does fill the bubble with water and rolls it around the yard to give him a 'bath'.
Lisa thinks she is getting away with pretending to be a college student because she is going to poetry readings and night and then attending her second grade class at school in the day. While she is going to Springfield University, Milhouse, Martin and Database follow her. While Lisa is at a lecture about Itchy and Scratchy (specifically, the episode Butter Off Dead), they blow Lisa's cover and her college friends don't like her anymore. Lisa also loses her friends that are her own age because she went off with college kids.
Bart gets used to his bubble and uses it to become popular and it also amuses Homer. Bart tells Lisa that in order to get her friends back she has to pull a prank on Principal Skinner. A day later, while Superintendent Chalmers is dedicating a parking annex to Skinner for his years of service as "interim principal," Bart rolls Lisa while she is inside his bubble. He then pushes her off the school roof and she lands on the chocolate cake next to Principal Skinner. The chocolate cake splatters all over Principal Skinner. Lisa then gets her old friends back because of this and Bart develops agorophobia because he is outside his bubble and goes to live in an air vent, only to be sucked down as the credits start to roll.