Hurrah! It's holiday time, and the Famous Five are spending it at Billycock Hill. Most exciting of all, they've made a new friend—a real pilot! But when he disappears with top secret equipment, the Five are puzzled. Could their new friend be a spy?
The Five decide to bike to Billycock Hill, where Julian and Dick's school friend Toby lives on a farm. Toby has all the camping equipment the Five will need, so they can camp high up on the hill overlooking the countryside—and a nearby secret airfield.