Case History: While walking home from the movies, Shaggy and Scooby come upon an abandoned truck, its sole occupant an empty suit of armor. They summon the rest of the gang and examine the scene: there is an empty crate labeled as the property of Jameson Hyde White, Professor of Archaeology, England, and a delivery slip directing the contents to the county museum. They deliver the truck and its contents to the museum, whose curator, Mr. Wickles, tells them the legend of the Black Knight, which is supposed to come alive on nights when the moon is full. But he can't explain the disappearance of Professor Hyde White. Meanwhile, Scooby, snooping around, finds an odd-looking pair of spectacles; research at the library reveals that they are a type of magnifying glass made in England and used by archaeologists.

Deciding that there is something fishy at the museum, the gang return 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. But 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 it's time 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. But 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.

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