This film, in my opinion, is one of those influential masterpieces - ahead of it's time (1931). Lang created two film genres with this one film: Film-Noir, and the Crime-Psych Thriller.
A child murderer in Berlin, is terrorizing the public, inhibiting the lives of everyone from the Police, to the criminals, to innocent bystanders who are accused of being the murderous acts.
The most startling thing about this film is it's use of sound. M was Germany's first professional motion picture with sound, and is evident by the primitive sound recording. Characters can often be just barely heard, but regardless of the limitations of the technology in the medium, the use of sound is advanced even by today's standards.
Other cool tricks used by the Fritz Lang include heavy use of shadows - largely in the style of the yet to come Film-Noir genre, and the use of setting to create darker moods.