Mephisto Waltz, Part 1 (S04E01)

Vienna, 1909. Oskar is baffled when the director of security agrees to a secret rendezvous with a notorious, recently captured criminal. When the meeting is ambushed, he is compelled to seek the identity of a high-powered traitor who threatens the very existence of the empire. He, of course, needs the help of Max, who has recently returned from a very successful tour of America. (BBC iPlayer)