Sky pirates

When their model plane crashes at the Aero Club, Mike and Harry are befriended by ex-pilot, Charlie Bradford, and his niece, Maggie, who teach them to fly radio controlled planes. Charlie invites them to the test flight of his new long distance aircraft which is later stolen. When they learn that a diamond has been stolen from the French State Museum, they realise the plane is to be used to smuggle the diamond out of the country.