French thieves Gaston and Pierre plan to break into Castle Duckula to steal a priceless vase. They are caught, and inform Duckula that the vase is actually part of a map that leads to the treasure of Mandarin leader How Fat. Duckula goes in search of it and ends up on the island of Moi Chow in a race against How Fat and the mutinous penguins to the treasure.