Anne, Julian and Dick are going on holiday and for the first time they are going to visit their Aunt Fanny, Uncle Quentin and their tomboy cousin Georgina (or George) at Kirrin Cottage. But Uncle Quentin is busy working on a secret project. However a stranger by the name of Mr. Curton shows up and seems a little too interested in Uncle Quentin's project. Then, when Uncle Quentin goes missing, George must get together with her cousins in an attempt to try and save her father.
During a heavy storm, a tree fell on Kirrin Cottage - meaning with the cottage out of action, the Famous Five travel to stay with Mr. Lenoir - an acclaimed scientist and colleague of Uncle Quentin, who lives at Smuggler's Top. Lenoir has developed a plan to drain the surrounding swamps - bit Mr. Barling, a nearby rival, wants to stop him, so he kidnaps Uncle Quentin and Sooty, Mr. Lenoir's son. It's up to the Famous Five to get them back.
The four children and Marybelle, Sooty's sister, suspect Block, the deaf butler, to be involved in the kidnappings. George, Anne. Julian and Dick begin their search for Sooty and Uncle Quentin in the caves underneath the house and find adventure along the way.
George and Timmy are at Julian, Dick and Anne's house for the holidays as Uncle Quentin is embarking on another secret project and needs silence at Kirrin Cottage. The children are spending their time, adventure free and lazing in the garden wondering what to do, when suddenly a circus arrives in town. Sinister members of the circus are less than please by the presence of the kids as their caravans hide a secret and they will stop at nothing - not even murder - to keep it a secret.
Despite warnings from a Mr. Andrews, Julian and Dick sneak back to the old goods station - but this time catch a glimpse of a spook train. Convinced something sinister is behind the trains they decide to revisit the station in daytime. But then they bump into Mr. Andrews and his men - and they look like they are up to no good.
The Five have the weekend off from boarding school and decide to go on a hike o the moors. But the five split up when Timmy has an accident and George and Julian take him to a nearby vets, whilst Dick and Anne head off to the next stopping point. However, Dick and Anne take the wrong path, and as night draws in they persuade an old woman to take them in for the night. Dick is woken in the middle of the night, when a man appears at the window, shouting his name and a mysterious message. When the stranger disappears he leaves behind a map - which the five, when reunited...
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.