M is a 1931 German mystery thriller film directed by Fritz Lang and starring Peter Lorre as Hans Beckert, a serial killer who targets children, in his third screen role. Both Lang's first sound film and an early example of a procedural drama, M centres on the efforts of both a city's police force and its criminal syndicates to apprehend a serial child-murderer.
The film's screenplay was written by Lang and his wife Thea von Harbou. It features many cinematic innovations, including the use of long tracking shots and a musical leitmotif in the form of "In the Hall of the Mountain King", which is repeatedly whistled by Lorre's character.