Lisa has a big butt and her friends at school tease her about it (for seemingly no reason other than to embarrass Lisa), 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. 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 either forged the test or cheated, but discovers that the watermark of the test is real and that all the answers are correct. But he is not easily willing to give up, so Homer and Marge speak to Bart's teacher Mrs. Krabappel, who confesses that she forgot to fold up the map during the test, so therefore, everyone in the class (including Bart) received a 100. Bart gets his "party", which is attended by Aunts Patty and Selma, 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 children don't appreciate her anymore, so when she finds Nelson eating tadpoles from a fountain at the zoo 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 this out and does not want Marge giving them any "charity". 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, eats an entire Labor Day cake that was baked especially for 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 find Nelson's father, who 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 gotten 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 and Homer declares that everything is back to normal. However, Lisa tells Homer that, despite Nelson beating up the girls who made fun of her butt and eating something after days of starvation, she still thinks her butt is too fat and (like many women and girls in real life) will never be content with her body image. Upset over the news, Homer goads Lisa into admitting that she is happy about her weight, which Lisa refuses to admit.