When Mystery Solvers Magazine holds its first annual Mystery Solvers Contest in London, Fred Jones, Daphne Blake, Shaggy Rogers and the dogs attend. Along with other detectives, they must solve The Mystery of the Blue Carbuncle, Sherlock Holmes' last unsolved case. But they are not alone: Soon the ghost of Sherlock Holmes is following the gang around, and it seems he is trying to help them, but is he? When Sherlock steals security system blueprints, and Shaggy and Scooby are believed to have done it, they must clear their names before it's too late.