George and Anne are spending their holiday at a riding school called "Captain Johnson's Riding School" by themselves. There, George meets a girl called Henrietta who is similar to her in that she likes being a boy. One day, George and Anne hear about gypsies across "Mystery Moor" and become very curious about it. But when Julian and Dick decide to come to the stables and hear about the gypsies, the fun begins. The four children meet a blacksmith who tells them a story about "Mystery Moor". The four children decide to follow the gypsies and camp on the moor. There is something strange happening on "Mystery Moor". The Famous Five risk treacherous mists to follow the gypsies across the moor—but will the trail lead to danger?