The outbreak of World War I places Scots officer Geoffrey Richter-Douglas in an uncomfortable position. Although his allegiance is to Britain, his mother was from an aristocratic Bavarian family, and he spent his summers in Germany as a child. When Geoffrey is approached by a German spy who offers him a chance to defect, he reports the incident to his superiors, but instead of arresting the spy they suggest that he accept her offer - and become an Allied agent. In Germany, among old friends, Geoffrey discovers that loyalty is more complicated than he expected, especially when he finds himself aboard the maiden voyage of a powerful new prototype Zeppelin, headed for Scotland on a secret mission that could decide the outcome of the war.