|Illustrator||Eileen A. Soper|
|Series||The Famous Five series|
|Genre||Mystery, Adventure novel|
|Publisher||Hodder & Stoughton|
|Media type||Print (hardcover and paperback)|
|Preceded by||Five Have Plenty of Fun|
|Followed by||Five Go to Billycock Hill|
George decides to go camping with her dog, Timmy, so he can recover from an ear injury without being mocked for wearing a large cardboard collar that prevents him from scratching his wound. George is pleased to be joined at the campsite by her cousin Anne, but is disappointed upon learning that Anne's brothers, Julian and Dick, are visiting France.
George and Anne encounter a boy, the son of an archaeologist, and his small, one-eyed mongrel dog called Jet. The boy is excavating an old Roman camp to search for artefacts and asks the girls not to disturb him. Later that day, the boy's twin brother comes to their campsite, but the girls mistake him for the first boy, unaware they are dealing with twins. That night, Anne gets up for a drink but ends up near a derelict, ruined cottage, where she sees lights and hears whispers and footsteps. She then takes George and Timmy to the cottage but there is no indication of any human activity.
The next day, the girls again encounter the twins separately. The girls then go to George's parents' house for more food supplies and are informed that Julian and Dick will be arriving in a day or two. The following night, a storm prompts the girls to shelter in the old cottage, where they are shocked to see people outside.
Julian and Dick arrive the next day and the four children decide to stay in the ruined cottage. They subsequently discover a secret tunnel, a gang of thieves and a stolen blueprint which has only 2 copies in the world, one with Uncle Quentin and the other with Sir John Lawdler (which has been stolen).