During World War I, an American secret service agent, Evelyn Erith, opens and deciphers a coded letter. It says the Germans believe that Kay MacKay, an American concentration camp escapee, knows The Great Secret.
And that MacKay must be eliminated.
Erith finds McKay, saves him from German agents and convinces him to join U.S. Intelligence. Together they venture undercover into the Swiss Alps to discover and expose The Great Secret. The future of Europe depends on it!