Last time, Captain Midnight, Chuck Ramsay and Ichabod Mudd discovered the secret exit from the Nazi spy headquarters, behind the safe deposit vault beneath the tower building. Although none of the spies were captured, Captain Midnight, through the French refugee boy Henri Brouvard, knows that the ace Gestapo agent Baron Von Karp is operating in the United States. After returning Henri Brouvard to his new American home, Captain Midnight and Chuck return to secret squadron headquarters.