“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 ax-wielding maniac hiding in her back seat!”
Otto is fired from his job driving the school bus, partially because he doesn't have a driver's license, and, after getting kicked out of his apartment, goes to live with the Simpsons.
Bart and Milhouse attend a Spinal Tap concert, but it turns into a major flop and degenerates into a riot that was positively serious to get the Springfield SWAT division involved. 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 purchase an electric guitar for Bart, but he 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 blocking 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 Lou 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 stranded.
Matters quickly go downhill for Otto. He fails every segment of his driver's test at the Springfield DMV, misspells 'bus' on his written exam, and asks Patty if she was transsexual. He is unable to find a new job, and he is evicted from his apartment for not paying his rent. Bart, feeling partly responsible for the situation, convinces his family to let Otto stay for a while, until he can find a new job, and he moves into the garage.
Otto soon makes a nuisance of himself as he becomes comfortable laying around the house and playing his - formerly Bart's - guitar at full volume. 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 agrees that he has to move on, but between her and Bart, encourage Otto to give the driving test one last try.
Otto and Bart go to the Springfield DMV. Outside, an unconfident Otto frets that Homer was right about him being a bum. Bart corrects Otto, revealing that Homer called him a sponge, and Otto is enraged, and determined to pass, bursts into the DMV and demands another chance. Patty is ready to turn him down when Otto tells her that he too dislikes Homer, and he wants to pass to prove him wrong and "staple his license to Homer Simpson's Big Bald Head". Patty quickly changes her tune, and not only gives him another chance, but helps him with his written test and overlooks many of his practical fails as they both make fun of Homer's crude behaviour, even offering to take him out to a bar for a drink.
The next day, Otto is back to work with a "PROBATIONARY" stamp on his licence 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 grateful to be back as principal.