Julian, Dick, Anne, George and Timmy the dog are camping on Billycock Hill. The see a airbase but are warned off by one of the guards. They meet Toby who leaves nearby who had been visiting his cousin Jeff who is a pilot. They later meet Jeff and Toby's brother ben who has a pet piglet called Curly.
The next morning there are news reports Jeff has disappeared apparantly stealing a top secret plane and everyone is shocked. The media later reports Jeff crashed the plane and died.
Toby refuses to believe that Jeff was a spy, as he had always seemed a trustworthy man. The Five attempt to comfort the distraught Toby.
Later Curly, appears with a message written on his back which leads to the children finding Jeff imprisoned in an old house. The children rescue himm and the real plane thieves are exposed.