M is one of the high watermarks of Fritz Lang's career, and more broadly of German cinema from the Weimar era between World Wars I and II. Co-written by the director Fritz Lang and his wife Tea von Harbou, M traces the story of a child murderer, and was based on a variety of real killers who were terrorizing Germany at the time. The popular myth goes that the killer in M was based on Peter Kürten, popularly known as the Vampire of Düsseldorf, but Lang denied this up and down in interviews. Regardless of whether Lang was playing coy in his denial, the film nevertheless capitalized on the sensational Kürten murders and became a massive box office hit.
Lang initially planned to call his film Murderer Among Us, as an allusion to the rising Nazi party. Indeed, M seems inseparable from that historical moment, with its focus on the madness of crowds and the reflection of an ineffective government run on a bloated bureaucracy. The Weimar Republic collapse once inflation ran out of control, leaving Germany vulnerable to the Nazi party taking power. M depicts a precarious moment in Germany history, when the underworld wields more power than the state, and dissatisfaction with existing government structures runs deep.
The fate of Lang's own film career in Germany went arm-in-arm with the Weimar Republic's. When the Nazi party took power in 1933, propaganda minister Joseph Goebbels asked Fritz Lang to head its propaganda film department. As Lang recounts in an interview with Peter Bogdonavich, he told Goebbels that he'd gladly accept the author, and then fled to Paris that night. All the funds in Lang's bank account were frozen, and he was without a home. Watching M, this likely comes as no surprise. Lang masterfully balances two horrors: the child murderer on the loose, and the bloodthirsty crowds willing to throw conventional judicial justice to the wind for quick retribution in the streets. Watching M, one gets the impression that we're seeing not just a story about a child murderer, but a parable about fascism's swift ascent.
M fits into the Social Problem Film genre, one popular in both Weimar Germany and Hollywood at the time. It delves into the social ills that spur both serial killers and mob rule. Working in America, Lang would pursue the Social Problem Film genre further with his 1936 classic, Fury. The film also touches on Urban Thriller elements that typify Lang's smash hit Dr. Mabuse films as well as the German Expressionist tradition of silent film, what with the use of expressive lighting, shadow, and set design.
In his book The Films of Fritz Lang, film scholar Tom Gunning remarks on the unique narrative construction of M, explaining that instead of centering around an individual's struggle or a romantic relationship—as typical of many films at the time—M instead focuses on a whole city, with "the systematic nature of the search" and a "rational, goal-oriented process" in the city-wide hunt for the child murderer exposing "networks of communication and intersections."
M is also Lang's first sound film, and it combines an expert employment of visual storytelling emblematic of the finest silent film with savvy implementation of sound elements such as dialog, sound effects, and, memorably, a whistled melody.