Lucky Luke chases master-bank-robber Black Peppermint who left his nickname-memento. This brings him to Slumberville, a town which treasures its weirdness, including a reverend who keeps crashing onto the roofs of saloon, church or bank in flying machines financed by baron von Flaps, who hopes for a big military order from Fort Large. When the reverend breaks a leg, only Lucky volunteers as stand-in test-pilot, soon finding out the real criminal intent...