After Mr. Burns gets teased about his old car by the kids at Springfield Elementary School, he sends Homer to pick up a brand new Lamborgotti Fasterossa car (a parody of the Lamborghini Gallardo) in Italy. The family flies over on Alitalia, and have a great time touring the country, despite Homer and Bart's mockery of the culture and history of Italy (causing an embarrassed Lisa to pretend to be Canadian so no one will assume that she is part of the family, though this backfires when Homer brings in an American flag to smooch off the other passengers). After two huge wheels, one made of Mortadella and the other of cheese lands on their car and crushes the hood, they slowly push it into a small (fictional) Tuscan village nearby called Salsiccia (sausage), and are told that the mayor speaks English.
The Simpsons are shocked to find out that the mayor is none other than Sideshow Bob, who's equally shocked to see them. He explains that after he last attempted to kill Bart, he didn't return to jail and wanted a new life away from Springfield. Bob decided to get a fresh start elsewhere by "randomly" settling on a new destination in Italy (but only after passing over Orlando, North Korea, Shelbyville and, ironically, "Bartovia"). After a rough start, the Italians warmed up to him when he helped them crush grapes into wine using his enormous feet. After that, they elected him as mayor of their tiny village. As a result, Bob no longer has any intention of killing Bart, and he reveals that he has a family. He introduces them to his wife, Francesca and his son, Gino. They know nothing about his past life in America. Bob begs the Simpsons not to tell anyone, and they agree in order to have the car fixed.
Bob hosts a farewell party in the village for the Simpson family. However, that goes awry when Lisa gets intoxicated on wine and starts to spout off about him being an attempted murderer. He leads her away from the table, but as she stumbles backwards, she rips off his suit to reveal his prison uniform. The village finds out that Bob is a robber and attempted killer, and they sack him as Mayor. The Simpsons take off in the fixed car, and Bob swears a vendetta—not only on Bart, but now on Homer, Marge, Lisa, and Maggie Simpson, as well. The family flees, and Bob follows them on a motorcycle (a Ducati 999). Homer drives into a ditch and onto a Roman aqueduct, eventually landing on top of Trajan's Column in the Roman Forum. Bob's wife and son meet him, and though Bob initially promises to give up his vendetta, Francesca tells him, "If the Simpsons have dishonored you, they've dishonored our family. We take revenge as a family."
Meanwhile, the Simpsons are wondering what they should do next, since they are in a foreign country, have no car and no money whatsoever. Lisa spots a bus with a poster advertising Krusty the Clown's performance in Pagliacci. They meet up with him at the Colosseum in Rome, and he puts them in as unnoticed extras. However, Bob, Francesca, and Gino find them and corner them on the stage while Krusty, who went through a trap door, flees the stage, allowing Bob to perform the climax of Vesti la giubba. Before Bob and his family can finish off the Simpsons, though, Krusty's limousine picks them up; Krusty needs them to smuggle an ancient artefact back to America. The Terwilligers are disappointed at first, but then walk away, grinning maliciously and plotting revenge together.