Biggles Secret Agent

Written by W. E. Johns

Copyright 1940

A single adventure serialized as Castle Sinister over 10 issues of Modern Boy, #78 - #87, August 12, 1939 to October 14, 1939.
Professor Max Beklinder, inventor of an explosive called Linderite, has disappeared. He had been working on a secret poison gas for the British when word comes he had dies in a car accident in the European nation of Lucrania. Biggles is asked to head there to learn the truth.