At an abandoned water park, Bart and Lisa contract a bizarre infection, requiring treatment from steroids that cause temporary weight gain. Before they go back to school, Marge says "Someone's getting chunky" to Lisa in affection, which causes her to be insecure about her weight.
Meanwhile, after telling them he's been taking steroids, Jimbo, Dolph, Kearney and Nelson take Bart to an underground basement gym at the school where they teach him to get into shape and become a man.
Marge takes Lisa to the mall, shopping for back-to-school clothes to make her feel better. She later says "Flattering", making the word "Chunky" grow bigger in Lisa's mind. Lisa gets cranky in front of the customers and workers, so Marge takes her home.
Homer asks Lisa what's wrong. He lets Patty and Selma take her to the park where they describe how they no longer care what people think of them. Later, Marge apologizes to Lisa about what she said to her, but says words like "Normal" and "Perfect" to Lisa, making "Chunky" grow bigger and causing her more frustration. In the piano store, Luann tells Marge about a hypnotherapist named Dr. Wendy Sage, who is a woman who had a mastectomy and chose to stay flat rather than endure breast reconstruction surgery.
Meanwhile, Bart arrives in the gym, but he sees the guys hanging around with their girlfriends. Feeling that they haven't been friends with him, he runs away from the basement and plays basketball with the other children.
Lisa and Marge visit Dr. Wendy Sage. She uses hypnotherapy on them, transferring them to Lisa's mind. They see the "Chunky" word which takes up the entire space of her mind. They then travel to Marge's mind. At Marge's childhood memory, they discovered that Marge's mother had said a hurtful word: "Plain", to her. The word has been in her head for years. Marge and Lisa now know each other's feelings and love each other, thus, making "Chunky" and "Plain" shrink.
During the end credits, it is shown that there are hurtful words in everyone's heads (possibly said by their parents), including Comic Book Guy, Skinner's mother, Chief Wiggum and Moe.