Siblings Julian, Dick and Anne are spending the Easter school holidays at Kirrin Cottage with their cousin Georgina (known as George) and her dog, Timmy.

After George's parents, Quentin and Fanny, depart for some scientific conferences, the Five embark on a cycling and camping holiday.

He is later chased by a car driven by one of his wealthy father's former bodyguards called Rooky, who was fired because Richard had told tales about him.

When one of the gang members leaves in their car, Richard hides in the boot, narrowly escaping being recaptured and then goes to the police.

When the police arrive at Owl's Dene, Julian shows them the secret room containing an escaped convict and stolen diamonds.