The Famous Five are holidaying at a Riding School, when a group of gypsies traveling by turn up with an injured horse. Talking to Sniffer, the boy whose horse has a bad leg, they hear the gypsies are traveling to Mystery Moor. The gypsies seem in a hurry to leave, and when the five follow, they are told to "clear off and leave us alone." What can possibly be the secret of Mystery Moor? It's up to the Famous Five to find out.