After watching the latest McBain movie, Grampa Simpson suffers a case of a mild heart attack 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 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 the 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 American). He is very rich, but is quite 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 in Detroit.
Bart, Lisa, and Maggie are enthralled by Herb's wealthy lifestyle (although 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 would amount 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 of 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 be becoming spoiled. Herb reassures her they will be gratified and wonders how Homer is holding up.
Later on, Herb gets a call from his lead engineer asking him to come down to Powell Motors at once. He voices out concerns about Homer because he's causing them trouble by asking for items unsuitable in designing a car and even ignoring their suggestions when they tried to get him to think practical. The latest complaint came when he rejected one of the engineer's drawing of the revised plans for the car by ripping it up and drawing his own monstrosity. The lead engineer demands that Herb deals with Homer at once or else the engineers will quit. He refuses to hear any of it and demand they treat Homer right because he is his brother.
At the unveiling of "The Homer", Herb is horrified to discover that the car is a "monstrosity" that costs $82,000. Herb's company bankrupts and is taken over by Kumatsu Motors (a Japanese car company), his mansion is sold off and he leaves regretting that he ever met his brother. As he departs Detroit on a bus, Homer tries to apologize. However, Herb refuses to forgive Homer and angrily remarks that as far as he is concerned, 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 was a Simpson. When a healthy Abe comes over from Springfield to meet with Herb in a cab, he discovers to his disgust that Homer ruined his brother's life by bankrupting him by creating a car that is a disaster. Abe decries Homer for screwing up everything and declares he "knew [he'd] blow it", heading back to Springfield in the same cab rather than in the car with the rest of the family. In the end, while Homer drives the family home, Bart tells him that the car he built was great. He becomes relieved that at least one person he cares about seems to like it.