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,[2] M centers on the efforts of both a city's police force and its criminal syndicates to apprehend a serial child-murderer.

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.

Elsie's place at the table remains empty, her ball rolls away through a patch of grass, and her balloon gets briefly caught in the telephone lines overhead before blowing away in the wind.

Inspector Karl Lohmann, head of the homicide squad, instructs his men to intensify their search and to check the records of recently released psychiatric patients, focusing on any with a history of violence against children.

They stage frequent raids in seedier parts of the city to question known criminals, disrupting organized crime so badly that Der Schränker ("The Safecracker") summons the bosses of Berlin's Ringvereine to a conference to address the situation.

[11] The police search Beckert's rented room, find evidence there connecting him to both the letter and a past crime scene, and lie in wait to arrest him.

By falsely claiming that one of the watchmen was killed during the break-in, Lohmann tricks Franz into admitting that the gang's only motive was to find Beckert and revealing their plans for him.

His "lawyer" points out that Der Schränker, presiding over the proceedings, is wanted on three counts of manslaughter, and that it is unjust to execute an insane man.

[21] While researching for the film, Lang spent eight days inside a mental institution in Germany and met several child murderers, including Peter Kürten.

[22] Peter Lorre was cast in the lead role of Hans Beckert, acting for the film during the day and appearing on stage in Valentine Katayev's Squaring the Circle at night.

[25][26] Lang denied that he drew from this case in an interview in 1963 with film historian Gero Gandert: "At the time I decided to use the subject matter of M, there were many serial killers terrorizing Germany—Haarmann, Grossmann, Kürten, Denke, [...]".

[30] Likewise, the practice of the Ringvereine shown in the film of providing financial support for the families of imprisoned members was also based on reality.

[30] The Ringvereine, which were officially wrestling associations that existed for the physical betterment of German men, always sought to promote a very 'respectable', almost middle-class image of themselves.

[32] Though the Ringvereine were known to be gangsters, their hierarchal structure and strict discipline led to a certain popular admiration for them as a force for social order unlike the psychopathic serial killers who murdered random strangers for reasons that often seemed unfathomable, sparking widespread fear and dread.

[32] In an article originally published in Die Filmwoche, Lang wrote that the crime scene in Germany was "such compelling cinematic material that I lived in constant fear that someone else would exploit this idea before me".

[32] Adding to the debate was popular interest in the new science of psychiatry, with many psychiatrists arguing that crime was caused by damaged minds and emotions which could be cured.

[32] In addition, for many conservative Germans, the Weimar republic was itself born of crime, namely the November Revolution of 1918 which began with the High Seas Fleet mutiny.

According to this viewpoint, its origins in mutiny and revolution made the Weimar Republic an illegitimate state that could not maintain social order.

[37] The film was one of the first to use a leitmotif, a technique borrowed from opera; it associates a melody with Lorre's character, who whistles "In the Hall of the Mountain King" from Edvard Grieg's Peer Gynt.

[23] As with many other early talkies from the years 1930–1931, M was partially reshot with actors (including Lorre) performing dialogue in other languages for foreign markets after the German original was completed, apparently without Lang's involvement.

[43] In 2013, a DCP version was released by Kino Lorber and played theatrically in North America[44] in the original aspect ratio of 1.19:1.

"[23] Graham Greene compared the film to "looking through the eye-piece of a microscope, through which the tangled mind is exposed, laid flat on the slide: love and lust; nobility and perversity, hatred of itself and despair jumping at you from the jelly".

The site's critics consensus reads: "A landmark psychological thriller with arresting images, deep thoughts on modern society, and Peter Lorre in his finest performance.

[52] It is listed in the film reference book 1001 Movies You Must See Before You Die, which says: "Establishing conventions still being used by serial killer movies, Lang and scenarist Thea von Harbou intercut the pathetic life of the murderer with the frenzy of the police investigation into the outrageous crimes, and pay attention to issues of press coverage of the killings, vigilante action, and the political pressure that comes down from the politicians and hinders as much as encourages the police.

[24] Argentine noir classic "El Vampiro Negro", released in 1953 and directed by Román Viñoly Barreto, is based on Lang's original script.

[60] In 2015, Joseph D. Kucan adapted the screenplay into a theatrical stageplay entitled A Summons from the Tinker to Assemble the Membership in Secret at the Usual Place for production by the Las Vegas-based theatre company A Public Fit.

The play is environmental in nature, transforming its audience into the members of the criminal underground who have captured - and will judge - the elusive serial child murderer.

The play is primarily a courtroom drama, presented with no fourth wall, and utilizes flashback sequences to tell the story of the man's detection, capture and confession.