Anne's brothers Julian and Dick come to join the girls and initially mistake Henry for a boy, much to George's chagrin.

The children encounter a group of gypsies determined to visit a desolate place called Mystery Moor.

The Famous Five follow the gypsies to the moor and discover they are involved in receiving smuggled American banknotes, which are later revealed to be forgeries from France.

George and Anne are taken prisoners and held in a cave, but are rescued by Henry and a boy from the riding school, William.

A gypsy boy named Sniffer assists their escape, and George promises to reward him with a red bicycle and living in a house with a family.