This is a documentary on the true murders and the hunt for the killer Peter Kürten, on which Fritz Lang's 1931 film "M" is based, the introduction of profiling of a killer and what make him one that revolutionized police methods, as well as a look at the police work then and today; details and background on Lang, Lorre and on what happened to them and the film after being banned 1934 by the Hitler regime.