The Traitor (1936)

Original title: Verräter

This 1936 film tells the story of the doings of foreign agents in Germany and their allies among the German population. Having placed an ad in a local paper looking for "contact with bigwigs in German industry", enemy agents Morris and Geyer end up making contact with an engineer named Brockau. Brockau has developed an improvement in turning oil into gasoline and that's just what these enemy agents are looking for. Brockau, for his part, needs money, because his girlfriend is a selfish cow who demands more and more toys and trinkets, which has put our naive little nerd deep into debt. Brockau, however, is not the only unwitting maroon to fall into the clutches of the evil agents: the former bank agent Hans Klemm, now doing his service in the Wehrmacht, ends up being contacted by an agent from the other side and ends up getting blackmailed into working for them.