A young Englishman is posted to an infantry battalion in France late in the winter of 1917-1918. After some months in the trenches he is attached to the Royal Flying Corps. He is wounded and falls in love with his nurse. Once fit again he returns to France where he briefly encounters a German pilot—a prisoner. By chance they meet again in the 1920s, at a time of desperate chaos for Germany. In 1940 their paths cross once more. The Battle of Britain is over but the Nazis still entertain hopes of invading and defeating Britain. The Englishman is now a civilian police inspector. He stumbles on a German plan to prepare the way for invasion.Its organisation and execution has been entrusted to his former acquaintance—the prisoner of 1918.