Great Britain

When their model plane crashes at the Aero Club meeting, MIKE and HARRY are befriended by ex-pilot CHARLIE BRADFORD and his niece MAGGIE, who teach them to fly radio-controlled planes. CHARLIE repairs the boys' plane and invites them to the test of his new long-distance aircraft but the plane is stolen. They learn that the 'Napoleon' diamond has been stolen from the French State Museum, and realise that the plane is to be used to smuggle the diamond out of the country.