When an old, haunted house known as Grimsley Mansion opens to the public, the Scooby gang investigates, not only to locate a treasure in gold left by the house's prospector-owner, but to solve the mystery behind a legendary "Black Knight" who roams the corridors at night to keep visitors away. It's the gold that everyone's after, including the ghost himself, who turns out to be the Black Knight, a haunted suit of armor that Augustus purchased when he first built the house.