Rarely do magicians reveal their professional secrets. Consequently Frank and Joe Hardy are amazed when a well-known magician, the Incredible Hexton, offers to reveal the secret of his "Vanishing Man Act" and invites Mr. Hardy to be the subject. When their detective father fails to reappear, his sons are convinced that something sinister is afoot, despite Hexton's insistence that Mr. Hardy is playing a joke on them. While desperately searching for their father, Frank and Joe find themselves working with SKOOL, a U.S. organization of crack secret agents pitted against UGLI, an international ring of spies stealing government secrets from the U.S. and other democratic countries. In a lighthouse off the coast of New England and in Scotland, Frank and Joe and their pal Chet Morton grimly match wits with UGLI's evil agents. With cool daring the three boys invade the magician's Scottish castle, where an astounding surprise awaits them.Tthe young detectives' gripping adventures culminate in a dramatic climax when they unmask the secret agent on Flight 101. - Flyleaf.