A Gaggle of Galloping Ghosts

When the kids visit Franken castle, a castle transported to the states brick-by-brick from Transylvania, they discover it is haunted by a werewolf, vampire, and a monster similar to Frankenstein's. They also discover that when Mr. Franken died in the 1600's, he left a fortune in jewels behind. They investigate to see if the monsters and jewels are related.