He spied on a famous Boer spy , Capt. Fritz Joubert Duquesne , who became a German spy in both World Wars and claimed to have killed Field Marshal Kitchener . Much of it involved a famous Boer spy , Capt. Fritz Joubert Duquesne , who became a German spy in both World Wars and claimed to have killed Field Marshal Kitchener while en route to meet with the Russians .