Hugh Thomson, a surgeon-major, is engaged to Geraldine Conyers. He is ostensibly in command of military hospitals on the French front, but in reality, he is the head of military counter-intelligence. He is on the trail of a German master spy who appears to be able to go across borders and between Germany and England. Captain Granet is a wounded war hero who was recently awarded the DSO and is recovering after being caught and escaping from the Germans...