Scary Godmother is the whimsical all-ages story that follows the first trick-or-treating adventure of Hannah Marie, a young girl whose rotten older cousin is babysitting her one dark Halloween. Unhappy to be saddled with Hannah, her cousin cooks up a scheme to frighten her. But his scheme backfires when Hannah gets help from her Scary Godmother.

Mystery Inc. is summoned to investigate occurrences in a haunted villa, where a black knight terrorizes anybody who tries to get close to treasure hidden by the former owner of the building.