The Five are camping again, this time on Billycock Hill, which is near the farm of Toby, a boy from Julian and Dick's school. Toby was very proud of Jeff (his cousin) and thought he was a hero. When Jeff, a British Air Force pilot, flies off late in a stormy night with the newest aeroplane which is full of secrets, the Five are stunned. The media claims Jeff has flown away and people think he is a traitor. Could Jeff really be a spy? The Five and Toby disagree - Jeff had always seemed to be a trustworthy man. They are determined to find the truth.