Mystery Inc. has solved the granddaddy of all cases! They have tracked down Coco Diablo, the leader of a legendary costume criminal syndicate. Mystery Inc. believes that they can finally take a break now that Coco and her cat are incarcerated. Wrong! Halloween is threatened by the sudden appearance in Coolsville of ghostly doubles of the Scooby-Doo gang and other famous villains. Now it's up to the mischievous youngsters to unmask the next villain and save Halloween!