After watching the latest McBain movie, Grampa Simpson suffers a case of a mild myocardial infarction while complaining to the theater's manager. This prompts him to confess a long-hidden secret; Homer has a half-brother.
As Grampa explains, he had met a carnival floozy and prostitute before marrying Homer's mother. They had a son, and left him at the Shelbyville Orphanage. Determined to find his half-brother, Homer and his family go to the orphanage and find out that Abe's son was adopted by a couple named Mr. and Mrs. Powell and named Herbert.
Herb Powell (who looks just like Homer, except with more hair and a little less gut) is the head of Powell Motors car company (in danger of being taken over by the Japanese because of otherwise poor management and the use of names from Greek Goddesses on cars that doesn't appeal to the average Americans). He is positively wealthy, but is quite unhappy not understanding 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 in Detroit.
Bart, Lisa, and Maggie are enthralled by Herb's wealthy lifestyle (though Marge constantly worries about spoiling her kids), and Herb thinks that Homer, being an "average" American, is the perfect person to design a new car for his company, which has been losing business due to foreign car manufacturers. Herb introduces Homer to his company's design team, who soon use his lack of vehicle knowledge to try and design 'their' perfect vehicle. Back in the hospital, Abe is informed by Homer about Herb's wealth and business. He laments in not keeping him over Homer, whom he knew amounted to nothing. Before he leaves, Abe warns him to behave himself and not do anything dumb that could ruin and humiliate Herb.
When Herb asks Homer how the car design is coming along Homer can't give him a straight answer. Herb feels that Homer is not being forceful enough with his ideas, and gives Homer a pep talk. Pretty soon, Herb has invigorated Homer, who returns to the designers and begins demanding weird items like bubble domes, fins and several horns that play "La Cucaracha". While Homer becomes more a businessman, Herb spends time with Marge and the kids, becoming more of a family man like Homer. Herb's money and prestige enables the kids to enjoy life in style, but Marge is worried they may becoming spoiled. Herb reassures her they will be gratified and wonders how Homer is holding up.
At the unveiling of "The Homer", Herb is horrified to discover that the car is a "monstrosity" that costs $82,000 (which is more than he has). Herb's company bankrupts, his mansion is sold off and he leaves regretting that he ever met his brother. As he departs on the bus to Detroit, Homer tries to apologize. However, Herb refuses to forgive Homer and remarks that he "has no brother". While Marge tries to console a depressed Homer, Lisa laments on Herb's life in how he was a successful businessman with a wealthy life until he discovered he is a Simpson. When a healthy Abe came over from Springfield to meet with Herb in a cab, he discovers to his detest that Homer ruined his brother's life by bankrupting him in creating a car that is a disaster. Abe decries Homer for screwing up everything and declares he has no son after this washout and goes back to Springfield in the same taxi, rather than in the car with the other Simpsons. In the closing scene, while Homer drives the family to Springfield, Bart tells him that the car he built was great. Homer became relieved that at least one person he cares about seems to like it.