After watching the latest McBain film, Grampa suffers a mild heart attack. Thinking he might die, he is prompted to confess a long-hidden secret: Homer has a half-brother. Grampa explains that he met a carnival prostitute before marrying Homer's mother, and they had a son that they left at the Shelbyville Orphanage. Determined to find his brother, Homer and his family go to the orphanage and find out that Grampa's love child was adopted by a Mr. and Mrs. Powell and named Herbert.
Herbert Powell, who looks just like Homer, except he is taller, slimmer and has more hair, is the head of the automobile manufacturer Powell Motors in Detroit, which is in need of new ideas. He is very rich, but is unhappy not knowing who he is and where he comes from. He is overjoyed upon hearing of his half-brother and invites the entire Simpson family to stay at his mansion. Bart, Lisa, and Maggie are enthralled by Herb's wealthy lifestyle and kind personality, although Marge constantly worries about spoiling her kids. Herb then decides that Homer, being an average American, is the perfect person to design a new car for his company.
Homer is given entirely free rein in the design, but is at first too timid to voice an opinion, as Herb's designers begin to design the car with their own ideas in mind. When Herb gets word of this, he gives Homer a pep talk that sends him back to the designers determined to build the car with all sorts of weird effects like bubble domes, tail fins and several horns that play "La Cucaracha". At the unveiling of the new car, Herb is horrified to find that the car is a badly-designed and hideously styled monstrosity that costs US$82,000, leaving Powell Motors bankrupt and out of business, while Herb's mansion is foreclosed and he loses everything he ever worked for. As he departs Detroit on a bus, he angrily remarks to Homer that he would have been better off if he never met him and that as far as he is concerned, he has no brother. While Marge tries to console Homer, Lisa agrees with Herb and laments on his life before he discovered he was a "Simpson". Grampa soon arrives and asks about Herb. Before Homer can even answer what happened, Grampa remarks to Homer that he knew "he'd blow it", and leaves in the cab he just arrived in, rather than getting in the car with the rest of the Simpson family.
While Homer drives the family home, Bart tells him that the car he built was great, and Homer becomes relieved to discover that at least one person liked it.