Spies Of The Western Front

Published in the Boy's Friend Library series, #661, March 1939. Originally serialized in 11 consecutive issues of Modern Boy magazine, Vol.2 #23 - #33, in 1938. Subsequently released in hardback as The Return of Grey Shadow in 1949.
Captain John Scott (Scotty to his friends), youngest member of the elite Secret Squadron of British pilots operating behind enemy lines in WWI, is sent to Hamburg to assist an ailing undercover agent. In fact the agent has been arrested and is scheduled to be executed. Scotty learns this when he is joined by Grey Shadow and a young woman helping him to first free the agent and then complete the man's mission.