After taking career aptitude tests (scored by a malfunctioning computer), Lisa discovers that the occupation she is best suited for is homemaker, while Bart's test shows that he should be a policeman. Lisa is heartbroken over the result and is determined to prove the test wrong. She consults a music teacher for his opinion, but he tells her that, having inherited her father's stubby fingers, she can never be a professional saxophone player. Lisa is therefore required by the test to spend the day doing chores with her mother Marge, while Bart goes on a ride-along with the police.
Lisa hates her role as a homemaker and, realizing that her future dreams have been shattered, loses interest in being a good student. Bart enjoys spending time with the police, and he even ends up stopping Snake Jailbird during a car chase. When Principal Skinner discovers Bart's new interest in law enforcement, he enlists him as a hall monitor. Bart starts handing out demerits to his classmates for minor infractions and has order restored to the school. Meanwhile, Lisa becomes a sulky, rebellious student with no interest in school. She hangs out with the bad girls in the bathroom, makes snippy comments at Miss Hoover, gets disciplined by Skinner, and her grades drop as a result. The girls are impressed with Lisa's recent bad behavior and offer her a cigarette. Though hesitant to accept, she takes it and tells them that she will smoke it in class.
One day, while she is in detention for another snippy comment, Lisa secretly steals all of the Teachers' Editions of the schoolbooks and reveals the teachers' incompetence. When the books turn up missing, it is up to Bart to find out who stole them. He finds the books in Lisa's locker and realizes she is the culprit. Closing her locker, Lisa confesses to the crime and reveals she stole them because she was rebelling against her chosen occupation as a homemaker. She also tells Bart that before he became a hall monitor, he used to enjoy the simple freedom of rebellion. He admits so, but also tells Lisa that even he had his limits and that she could face expulsion for her actions. When Skinner finds the books, he is overjoyed, but then asks who stole them. Bart takes the blame and returns to his life as a bad student and detention hall regular.
When Milhouse takes him to Skinner's office for punishment, Lisa asks Bart why he took the blame for her and not let her take the fall. He tells Lisa he did not want her ruining her future as she had the brains and talent to do what she wanted. Bart encourages her to keep pursuing her future dreams as a professional saxophone player and promises to be there for her when she needs his help again (as well as borrowing money from her). Lisa is touched by Bart's deeds and returns to her old life as a good student. As he spends his time in detention, Lisa plays her saxophone outside his classroom to comfort him. While hearing her play her sax, Bart makes encouraging comments that Lisa has a great future as a professional saxophone player regardless of what anyone else says.