The Family spends a night of playing Scrabble and Lisa reminds Bart that he's supposed to be stimulating his brain with various vocabulary if he hopes to pass his intelligence test. He cheats his way to victory by coming up with his own vocabulary, which ironically describes Homer. This angers his father and he spends time chasing after Bart, much to the embarrassment of his family.
At school, Bart is busted by Skinner thanks to class genius, Martin who noticed Bart's sloppy handiwork in misspelling wiener. Faced with the prospect of failing an intelligence test, Bart surreptitiously switches exams with Martin. When the school psychologist, Dr. Pryor, studies the results, he identifies Bart as a genius, to the delight of Homer and Marge, who enroll him in a new school. However, Lisa is not fooled by his supposed genius and still believes Bart is a moron. She's not the only one who believes this as Skinner also shares her belief, but he takes advantage of Bart's departure from Springfield.
At the Enriched Learning Center for Gifted Children, Bart feels out of place among the other students with advanced academic skills. Meanwhile, Marge attempts to stimulate Bart with a little culture by taking the family to the opera. However, this proves disastrous as Bart and Homer are quite disruptive, much to Lisa's joy. Ostracized by his brilliant classmates, Bart visits Springfield Elementary, where his old friends also reject him. On the bright side, he enjoys newfound attention from Homer and he covers for them when Marge makes another attempt to stimulate Bart's brain by taking to a ballet recital. After Bart's chemistry experiment explodes, filling the school lab with green goo, he confesses to Dr. Pryor that he switched tests with Martin. Dr. Pryor realizes that he was never a genius and has him readmitted back to Springfield Elementary.
Bart returns home and tells Homer that he cheated on the intelligence test, but that he is glad they are closer than ever. An angry Homer chases Bart through the house, only for Bart to lock the door of his bedroom. Lisa pronounces that Bart is back to being his normal, dumb self.