“So the lady drove faster, but a strange car kept banging into her from behind! Finally, the lady swerves off the road into the woods, and lost the other car. And that's when she realized that the man in the other car wasn't trying to hurt her. No, he was trying to warn her... about the axe-wielding maniac hiding in her back seat!”
Bart and Milhouse attend a Spinal Tap concert, but it turns into a major flop and degenerates into a riot. Nonetheless, Bart is impressed by the aging heavy metal band and wants to become a rock guitarist. Homer also unintentionally left Milhouse behind in the ruins of the concert, and had to double back to pick him up.
Homer and Marge decide to buy Bart his own electric guitar, but Bart struggles to learn how to play it. The next morning on the school bus, Bart brings his guitar onto the bus and Otto compliments him on his "cherry axe" to which Bart replies that he thinks the guitar is broken. Bart hands it to Otto, who – with the bus still stopped in traffic – wows his passengers with an impromptu concert, as well as severely irritating the people behind him. However, Otto loses track of time and is forced to drive recklessly to school. In the ensuing chaos, the bus runs Spinal Tap's bus off the road, and turns over onto its side in the town square. Numerous people call Springfield Elementary School to report the accident, earning Otto a reprimand from Principal Skinner. However, when Officer Eddie asks for Otto's driver's license to complete his report, Otto is forced to admit he doesn't have one (or any sense of identification, as he took the wrong underwear). Otto loses his job, and Skinner takes over his route. Skinner finds it hard going, not being an aggressive driver like Otto and spends nearly a day being stuck.
Things quickly go downhill for Otto. He fails his driver's test at the Springfield DMV (failing every segment and misspelling 'bus' on his written exam, and it is also implied that part of the reason why Patty failed him was out of revenge for his asking if she was transsexual), he is unable to find a new job, and he is evicted from his apartment. He calls upon the Simpsons to take him in until he can find a new job, and is allowed to move into the garage, after Bart convinces the family.
Otto makes a nuisance of himself as he becomes comfortable laying around the house and playing his guitar. Homer soon loses his patience with Otto after finding a clump of his hair in their bathroom sink. The last straw is when Otto tells Lisa a ghost story similar to High Beams from Scary Stories to Tell in the Dark. Marge and Homer hear Lisa's screaming and Homer demands that he be sent on his way. Marge insists that Homer let him stay, stating that they're the only family he's got. Homer replies,"I don't care. This is not Happy Days and he is not the Fonz." Marge and Bart, however, encourage Otto to give the driving test one last try.
Otto and Bart go to the Springfield DMV. Outside, an unconfident Otto states that Homer was right about him being a bum. Bart then corrects Otto by revealing that Homer called him a sponge. To which Otto becomes enraged and marches into the DMV. He demands another chance to take the test and Patty refuses. Otto refuses to give up and tells her that he wants to be retested so he can prove Homer wrong about him. This quickly changes Patty's demeanor after learning that Otto shares her spite for her brother-in-law and she helps him with the written test. Though Otto isn't successful in his retest, Patty decides to grant Otto his license anyway after he entertains her with stories of Homer's crude behavior and offers to take him out to her favorite bar for a margarita. Otto is back to work, although his license is marked "PROBATIONARY" when he shows it off to Bart. Otto drives off into the sunset, with Skinner remarking, "Hail to the bus driver, bus driver man" and is happy to be back as principal.