During Springfield's St. Patricks Day Parade, a Duff Beer float sprays beer into the crowd. Bart, holding a novelty horn to his mouth, catches most of the liquid and becomes very drunk. After images of Bart in his inebriated state are broadcast on TV, a group of women bursts into City Hall and demands an all-out ban on the sale of alcohol. Responding to their concerns, a clerk discovers a prohibition law that has been on the books for 200 years but has never been enforced. On the basis of this law, Mayor Quimby decalres an end to liquor sales.

Unable to stop gangster Fat Tony from smuggling alcohol into Moe's speakeasy, Chief Wiggum is replaced by Rex Banner, an Elliot Ness-type from Washington, who puts Fat Tony out of business. When Moe's liquor supply dries up, Homer steals Duff barrels from the city dump and masterminds a complex plan to bring the beer into the speakeasy.

"Beer Baron" Homer's beer supply is soon depleted, and Homer concocts his own "bathtub hooch" in the basement. Eventually, he ends his bootlegging career and approaches Chief Wiggum with a plan: the Chief will restore his good name by "exposing" Homer. As Homer is about to receive his punishment under the ancient prohibiton law by being catapulted out of town, he is saved by an impassioned speech by Marge. As Banner rebuts, he steps into the catapult and is cast away by Wiggum. Belatedly, the City Hall clerk announces that the statute had, in fact, been repealed. The townspeople celebrated by getting drunk.