All of Springfield gathers for a parade honoring Saint Patrick’s Day. During the festivities, a Duff Beer float sprays beer into the crowd. Bart inadvertently steps into the path of the oncoming liquid—and suddenly becomes very drunk. Images of the tipsy minor are broadcast on television, sending shock waves throughout the community. This prompts an editorial from Kent Brockman, who calls for a revival of prohibition. A group of temperance women burst into city hall, demanding an all-out ban on the sale of alcohol. An old city clerk discovers that a prohibition law has been on the books for 200 years, but was never enforced. The mayor decides to embrace the old law, and all liquor sales are ended.

Gangster Fat Tony senses a business opportunity and begins supplying Moe’s speak-easy with smuggled alcohol. When Wiggum fails to stop the rum-runners, he is replaced by Rex Banner, an Elliott Ness-type from Washington. Banner puts an end to Fat Tony’s operation, and Springfield’s liquor supply dries up. Undaunted, Homer digs up Duff beer barrels from the city dump. He pours the beer into bowling balls and, through an elaborate system of subterranean pipes, smuggles the precious liquid into Moe’s speakeasy. Surprisingly, Marge approves of her husband’s scheme, labeling it “the cleverest thing he’s ever done.”

When word spreads that the mysterious “Beer Baron” has outwitted Banner, the lawman vows to bring his foe to justice. Shortly thereafter, Homer’s beer supply runs dry. With Bart’s help, Homer concocts his own “bathtub hooch” in the basement. But the volatile bathtub stills suddenly erupt in an enormous explosion that rocks the house. Marge urges her husband to give up his scheme. Homer agrees it’s time to put an end to his days as a bootlegger. He approaches (the now unemployed) Wiggum with a plan. The twosome make it appear that Wiggum exposed Homer as the elusive Beer Baron. Those who break the prohibition law, however, must be punished by catapult. But before Homer can be shot skyward, the old clerk discovers the prohibition law was repealed 199 years ago. Homer is released.