Lisa has a big butt and her friends at school, Sherri, Terri and Janie let her know about it. Homer explains to her about the Simpson butt, it is something she'll have to learn to live with. Bart comes home with 100 on his test, and now he wants to collect on the party that was promised. Lisa is trying to deal with her weight issue, meanwhile Bart gets his party; with his relatives, Ralph, Martin and Milhouse via speakerphone. It's the worst party ever as far as Bart's concerned. It's made worse when Lisa returns home only to be confronted by her mother wanting her to eat a big slice piece of cake, she runs out of the room crying. Marge is bummed that the kids don't appreciate her anymore. She finds Nelson in the park and spends some quality time with him and when her kids continue to shun her, she adopts Nelson as a surrogate child. Lisa tries diet and exercise to reduce her big bottom. Marge brings Nelson home to do some chores around their home, but Nelson's real mother returns that money that Marge paid her son and then she leaves for Hollywood, leaving her son in Springfield and homeless. Nelson goes to the only place where he can find refuge, the Simpson home. Much to Bart's dismay Marge lets Nelson stay in his room. Late one night Bart finds Nelson singing for the return of his father and Lisa overindulging in a cake (she'd previously been starving herself). Lisa gets help from Nelson to get revenge on Sherri and Terri. When the pair returns to the Simpson home there is a surprise waiting for Nelson; Bart has found his father. Nelson is reunited with his father (and his mother who's returned from Hollywood with an acting job). Lisa still hasn't found a conclusive solution to her weight problem.