While walking home from the movies, Shaggy and Scooby come upon a abandoned truck with a mysterious suit of black armor inside. They summon up the rest of the gang and examine the scene: an empty crate that is labeled property of Jameson Hyde White, Professor of Archaeology, England, and a delivery slip directing the contents to the county museum. The gang delivers the contents to the county museum, where the curator, Mr. Wickles, tells them about the legend of the Black Knight, who comes to life on nights when the moon is full. However, he can't explain the disappearance of Professor Hyde White. While Scooby is snooping around the museum, he find a pair of odd-looking spectacles. After research at the library, these odd-looking spectacles are a type of magnifying glass made in England and used by archaeologists.
Deciding that something is going on at the museum, the gang returns to the museum that night. Shaggy breaks in through an upper window. as he goes to let the others in, eyes in an Indian effigy watch him. The gang splits up. The moon is once again full and the Black Knight is roaming the halls. He threatens an oblivious Velma (who near-sightedly mistakes him for Shaggy) and menaces Scooby after he takes an interest in the dinosaur exhibit. After escaping the Knight, Scooby and Shaggy notice that there is a painting missing from the walls, though it has returned by the time they fetch the others. Daphne notices a trail of paint which leads them to a mummy case. Opening the case, they find a secret room in which museum paintings are being duplicated. When they decide to fetch the police, the Knight shows up. The gang flees into the relic room, where Scooby and Shaggy hide in a World War I biplane. Shaggy tries to trick the Black Knight into leaving by using his ventriloquism, and it nearly works. Unfortunately, Scooby accidentally turns the plane's engine on. The ensuing chase and crash knocks out the Black Knight, who is revealed to be Mr. Wickles. It is revealed that Wickles and a gang of forgers had been making fakes of the paintings in the museum collection and stealing the originals. Wickles had crept into the suit of the Black Knight and kidnapped Hyde White, the only person who could have spotted the fakes. Scooby catches Hyde White's scent and leads them to the Indian effigy, inside of which they find the archaeologist gagged and tied up.