The town of Springfield celebrates Saint Patrick's Day by drinking a large amount of beer. At the parade, Bart buys a plastic novelty horn. When a Duff Beer float sprays free beer, most of it goes into Bart's horn, making him drunk. A group of angry ladies complain to the mayor they should ban all forms of alcohol. The keeper of the town laws goes over some old texts and discovers that a prohibition law has been around for 200 years, but has never been enforced. Quimby has no choice but to ban alcohol from Springfield. This news causes all of Springfield's alcoholics to faint.
All the beer barrels are buried in the dump and Duff goes out of business shortly after releasing Duff Zero (Thinking that their customers drink Duff because it tastes good and not simply to get drunk). The mafia begins providing beer illegally, buying off the police force. Even though a prohibition law has been passed, everyone goes to a nearby speakeasy, cleverly disguised as "Moe's Pet Shop", for beer and dancing. The ladies against beer enter the bar and see Chief Wiggum drunk and dancing. They demand they bring in a police chief who will enforce the law. They send a telegram to Rex Banner to get to Springfield ASAP. He totally eliminates all the beer and kicks Wiggum out of his job.
Homer, who dislikes the no beer methods of Banner, figures out a way to keep Moe's bar operating, through bootlegging. One night, he and Bart sneak out to the city dump to reclaim the beer that was disposed of when the Prohibition law began to be enforced. He then sets up shop in his basement pouring the beer into hollow bowling balls. Using an intricate set of pipes under the Bowl-a-rama, he bowls the balls into Moe's, and in return, Moe gives in money to Homer that will support the Simpson family.
Marge later catches him, but compliments him for doing so since it's the most brilliant thing that he's ever done over his years of stupidity. She also adds that the law is considered unjust that led to Homer's actions and is amazed of how much money Homer has made. Lisa, on the other hand, objects over this, but is unanimously sent to her room by the entire family.
The media realizes someone is allowing Springfield's underground alcohol trade to flourish, and they give the still-unknown Homer the nickname "Beer Baron". Rex investigates, but finds no evidence. He vows to catch the beer baron and soon resorts to stopping people in the street to question them.
Homer runs out of beer from the barrels, so he decides to start making bootleg hard liquor when Moe gets mad at him for the beer shortage, this proves difficult as he not only has to place a suspicious order for 42 bath tubs and that his stills keeps exploding. He sees Wiggum all pathetic-like and decides to help him in his situation. He gives his story to Wiggum so he can get his job back, however, Homer is punished for his illegal alcohol supply and he is going to be sent away by catapult.
Marge tells the town how silly the law is, but Rex interrupts saying that they don't get to decide which laws they follow and which they don't. They see him step on the catapult and Wiggum sends him "back to Mama". The old man reads the laws and discovers the law was repealed 199 years ago. The mafia fills the town with beer within 5 minutes (although promising 4 minutes), everyone is drunk and happy again.