M is a German thriller directed by Fritz Lang in 1931. It is one of the first modern serial killer movies ever created and remains a thought-provoking classic.

When nine children are murdered off the streets of Berlin, the public becomes frantic. The police begin a comprehensive investigation that disrupts the business of local gangsters. The gangsters want the police investigation to end, so they take it upon themselves to find the murderer. Both investigations are followed throughout the movie and the distinction between police and gangster starts to blur.

The gangsters first catch the killer Hans Beckert (Peter Lorre) and attempt vigilante justice. Beckert expresses his lack of control and disgust over his evil actions. Before the gangsters can deliver their verdict, the police discover them and arrest Beckert. The viewer is left pondering which type of justice Beckert deserves. The movie ends with the mother of one of Beckert’s victims warning other mothers to watch their children closely.

This was the first movie Lang produced that had sound. He applied techniques learned from opera to make the sound design impactful. Throughout the film, Beckert whistled the song, “In the Hall of the Mountain King”. The viewer learned to associate the whistling with the presence of the killer. This technique is commonly used in movies to this day.