British Intelligence (1940)
The only person I knew in this was Boris Karloff who played a questionable butler to a high up Government minister who is trying to find out the true identity of notorious German spymaster Strendler, who seems to easily obtain top secrets to pass onto the Germans during WWI.
The film then develops into not only double agents, but triple too, so it becomes difficult to work out who are the spies and who are the genuine British Intelligence.
Good film though that gets you guessing right up to the end, and like me you would probably get it wrong as to who are the baddies.
Well worth a look.