As Lisa puts the finishing touches on an elaborate hopscotch grid and leans down to pick up a pebble, some of the other girls make fun of the size of her rear end. Lisa returns home and tells her father what happened. Homer says Lisa has what’s known as “the Simpson butt.” Later Bart shows Homer his perfect test score…and demands the party he was promised. A suspicious Homer checks with Mrs. Krabappel, who confirms Bart did get a perfect score, as did everyone else who took the exam on state capitals, mainly because she forgot to roll up the map in the classroom. Meanwhile Lisa makes her way to the Springfield mall, hoping some new clothes will make her look thinner. The incredibly thin female mannequins only make her feel worse. The next day Marge throws Bart a party, but the guests turn out to be Grampa, Aunt Patty, Aunt Selma, Grandma Bouvier, Martin, and Ralph: hardly Bart’s idea of great party guests. When Lisa returns home, Marge offers her some party cake, but Lisa, obsessed with her weight, runs from the room, crying.

Marge reaches the conclusion that her children don’t appreciate her anymore, and would rather hang out with their friends; but she thinks she’s still got a whole lot of mothering left in her. While playing with Maggie in a park, Marge notices Nelson Muntz grabbing tadpoles out of a fountain and eating them. Feeling sorry for the boy, Marge gives Nelson Lisa’s lunch. Nelson is very grateful for the gesture. Later Bart and Lisa reject Marge’s offer to go bowling. Hurt, Marge decides to take Nelson bowling. Nelson is again appreciative of the gesture. Nelson tells Marge that his mom’s dream is to work at a strip club in Vegas, and his father went out to buy a pack of cigarettes one day and never came back. Marge offers Nelson the chance to make some extra money doing odd chores around her house. Later Bart starts to panic when he sees Nelson, who has a long history of beating him up, doing things around the house. Mrs. Muntz shows up at the house and tosses a handful of change at Marge. She says she doesn’t want her charity. Marge explains she was just trying to give Nelson some self-respect. Later Nelson shows up at the house and says his mother ran off to be an actress in Hollywood. Bart reacts with shock when he realizes he and Nelson are sharing the same bedroom.

Marge asks Bart to be nice to Nelson. Moments later Nelson pummels Bart after he refuses to eat his breakfast. Late one night, Bart discovers Nelson singing “Papa, Can You Hear Me?” in the living room. He realizes he’s got to do something to change the situation. Meanwhile Lisa loses all self-control and begins gorging herself on cake. When Nelson finds out that girls at school were mean to Lisa, he says he has a solution. He brings a skunk to school and lets the animal spray the girls who made fun of Lisa. Lisa thanks Nelson for his help. Later Mr. Muntz shows up at the Simpson house. He tells Nelson how he disappeared: while buying cigarettes at the Kwik-E-Mart, he accidentally ate a candy bar filled with peanuts. Terribly allergic, Mr. Muntz’s head began to swell up, and an evil circus ringmaster imprisoned him in a sideshow cage. Bart located Mr. Muntz and rescued him. Shortly thereafter Bart reunites Nelson with his mother, who landed a minor acting job after sleeping around with various directors. With the Nelson crisis resolved, Homer tries to help Lisa with her weight obsession, but she tells him it’s an open-ended problem.