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Willie the Weasel has created a formula that turns him into Mr. Hyde. While the Blue Falcon and Dynomutt are on the trail of the mysterious Mr. Hyde, they encounter the Mystery, Inc. gang and Scooby-Doo. Mystery, Inc. teams up with the Blue Falcon and Dynomutt (who happens to be Scooby's hero) to stop Mr. Hyde, who has threatened to turn the population of the entire city into "Mr. Hydes" unless he is proclaimed mayor