When Bart gets a perfect score on a state capitals test (thanks to Mrs. Krabappel leaving a map of the USA on the wall for all to see), the party Marge throws for him is so lame that Marge thinks that Bart and Lisa feel she is "overmothering" them, so she shares her love with someone else - Nelson - resulting in Marge taking Nelson in (and putting him in with Bart) when Nelson's mother leaves him; Lisa gets the impression that she has a big butt - from her family, friends, and mirror.
Lisa's friends at school tease her about her big butt, causing her to become self-conscious about her weight. Homer only makes matters worse when he tells her about the "Simpson butt", something that all Simpsons have. He goes on to draw himself the way an animator would, starting with circles, then filling in the rest of the details (there is also an inside joke here: Homer draws his hair and ear forming the initials MG "for some reason" as he narrates; see trivia note). Bart brings home a geography test with a grade of 100, eagerly expecting a party that was promised to him if he ever got a 100. However, Homer thinks Bart forged the test, and is not easily willing to give up. They visit Bart's teacher Mrs. Krabappel, who confesses that because she forgot to fold up the map during the test, everyone in the class received a 100. Bart gets his "party", which is attended by Patty and Selma Bouvier, Grampa (sleeping in an armchair), Grandma Bouvier, Ralph, and Martin. Milhouse can only attend via speakerphone because he has the measles. Bart does not enjoy the party and to make matters worse, Lisa runs up to her room crying when Marge offers her a big slice of cake.
Marge thinks her kids do not appreciate her anymore, so when she finds Nelson eating tadpoles from a fountain in the park, she decides to become a mother figure in his life and spend some quality time with him. She brings Nelson home and pays him to do some chores. Nelson's mother finds out and does not want Marge giving them any charity or trying to make him respectable, to which Marge retorts that she should go home to her son and "not have intercourse on the way home!" (The exchange shocked the family so much that Lisa was even willing to eat more carrots when she previously would barely even eat a piece of the skin.) Later that night, Nelson's mom has left town, and Nelson, having no place else to go, decides to stay with the Simpsons. Marge lets him sleep in Bart's room, forcing Bart to sleep on the floor, underneath his bed.
Late one night, Nelson is singing about his long lost father, while Lisa, unable to take any more starvation, is stuffing herself with an entire cake that says "Happy Labor Day, Lenny”. The next day, Nelson helps Lisa get back at Sherri and Terri for teasing her. Nelson unleashes a skunk, which sprays both of them, while Lisa and the rest of the children point and sing a modified version of "Jingle Bells." Both twins are horrified and run off screaming. When Lisa and Nelson return home, they meet Nelson's father, whom Bart found working at a freak show in a circus. It turns out that Nelson's dad had really gone to the Kwik-E-Mart (which Nelson has repeatedly stated previously in the series), where he had a severe allergic reaction from eating a chocolate bar full of peanuts. Coincidentally, the circus had made a stop in the Kwik-E-Mart parking lot, and found Nelson's father, thinking he was some kind of half human, half monkey beast. Nelson's mother finally returns, after going to Hollywood and getting the lead role in Macbeth, playing Lady Macbeth. At the conclusion of this episode, Nelson kindly thanks Bart for finding his father and goes back to live with his family. However, he does hint that although he owes Bart for having his family reunited, and he wishes to stop bullying him, he may not be able to. Lisa still has an unhealthy obsession with her weight, although Homer tells her to say that everything is fine.