M is significant both as director Fritz Lang's first non-silent movie and also as actor Peter Lorre's breakout performance. Released in 1931, it is acclaimed for its revolutionary use of audio within the context of editing, where it becomes a creative tool of storytelling instead of just a microphone for actors.
M is set in pre-Nazi Germany and tells the story of a town that falls prey to a mysterious child murderer who roams the streets. Hans Beckert (Peter Lorre) is a mentally troubled loner who wanders the city, nervously whistling a passage from Grieg's "Peer Gynt" suite and seeking out his young victims. After several missing children turn up dead, a pattern emerges that indicates a serial killer, although he remains elusive. The city is in hysterics as time drags on without a capture; the police are baffled, understaffed and begin a constant vigil of every corner of town. Even the criminals of the city band together to catch the killer, as his ramp-up of the police's watch makes their smaller crimes more difficult. It's never a mystery who the criminal is, and the pull of the movie comes from studying this horrific and troubled killer who both hides himself, but secretly wants to be relieved from his compulsion to kill. The subject matter is surprisingly dark for the period and touches on taboo topics that feel edgy even today.
More than just a villain and justice movie, M touches on several aspects of the psychology of culture and stability. The most striking is the growing hysteria of the masses. What starts as concern and prevention against the child murderer loose on the streets quickly turns into citizens turning on each other over misheard whispers of rumours. It's hard not to draw parallels with our own post-9/11 culture, where "innocent until proven guilty" has been flip-flopped in the name of prevention and security. The other interesting aspect of the movie is the very organised criminal underworld. Every area from safe-cracking to panhandling on the street operates as an established and largely expected level of society. But when the child murderer emerges, things tip too far on the impropriety scale, which hurts the "honest" criminals as much as anyone else. To this segment of the population, it's as if there is a function for crime in society, and if properly kept in balance is as much a part of daily rhythm as anything else.