In London, August 1914, Austrian star Elsa Duranyi (Gertrude Michael) and English matinee idol Alan Barclay (Herbert Marshall) are in love and plan an immediate marriage. But the War comes and Elsa mysteriously disappears. Alan's ease in speaking German results in his appointment to the British Intelligence and, to aid his use as a spy, they announce he was killed in action. He takes the name and personality of "shell-shocked" Hans Teller, a German prisoner, and is sent into Germany on an exchange of prisoners. Elsa, now a spy in the service of the Fatherland, is in Monte Carlo, where Allied officers on leave can be tempted into revealing war secrets. In Germany, Alan, posing as Teller, is listed as unfit for service, contacts Carl Schrottle (Rod LaRocque), another British agent. They are to locate the German "Big Bertha," the long-range gun bombarding Paris. They are successful and the gun is destroyed. Elsa is recalled and given the assignment of locating the British spy ...