There are only three Fritz Lang films in the Collection -- discounting his delightful appearance as himself in Godard's Contempt -- and these appearances are fairly spread out. We last saw from him with Spine 30 and will next see him at Spine 649. But for now we have Spine 231, his 1933 follow up and sort of sequel to M (as Otto Wernicke plays the same character in both): The Testament of Dr. Mabuse.
M had an interesting background in that Nazis tried to shut it down during pre-production despite their not having come to full political power and Lang's insistence that the film was not meant to be anti-Nazi. The Testament of Dr. Mabuse, however, was dedicatedly anti-Nazi and, well, the Nazis were many things, but they weren't really dense. The film was banned in Germany, not shown publicly in the country until 1961. It was the last film he made in Germany until 1959.