Hugh Thomson, a surgeon-major, and Geraldine Conyers are engaged. He is in command of military counter-intelligence while being ostensibly in charge of military hospitals on the French front. He is a German master spy who appears to be able to cross borders and switch between Germany and England. After being twice captured and again escaping the Germans, Captain Granet is a wounded war hero who was recently given the DSO. He meets Geraldine Conyers and eventually falls in love with her. The development and usage of secret military weaponry designed to attack German submarines drives the plot forward. Aircraft remain novelties, and a nocturnal Zeppelin raid takes place on a top-secret armaments laboratory. There are glimpses of the brutality of Ypres and other wars, and there is still a remarkable degree of fluidity between the French battlefields and London society.