Lucky Luke spots a sleeping prospector, and decides to play a practical joke on him, planting a little gold nugget in the pan. The prospector awakes to his greatest dream come true, and when Lucky Luke chases the euphoric gold digger and tries to reveal the joke, the prospector shoots at him, speeding into town to stake his claim and shout the news. The madness of a gold rush follows, bringing changes to Buffalo Creek of a greater scale than Lucky Luke expects, and when he makes a final attempt to explain the truth, he finds himself arrested. Unable to remedy the situation from inside a jail cell, Luke must let the situation run its course, a change coming undone as quickly as it started, as an expert examines the gold nugget and realizes that it was discovered in California fifty years ago. Buffalo Creek then empties of its inhabitants, becoming a ghost town.