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.