M is considered by most to be the first serial killer movie (with Peter Lorre as a child murderer), and Lang's first talkie. While he now has sound, Lang still uses the power to silence to great effect in the film with long wordless sequences of the police massing for raids, Lorre being followed by criminals turned vigilante (because all the police efforts to find the murderer is hurting their business), as well as his trademark wacky camera angles. There are so many deep shots down streets, stairways, hallways, holes in the floor, that the film might actually work if converted into 3D.
One of the great things about the film is that you're never sure what it's going to do next. After a spectacularly grim opening of a mother waiting for her daughter to come home (and she never does) it become a police procedural as the cops pull out all the stops to solve the case with no clues and no real witnesses. There's an amazing sequence with the camera bouncing back and forth between identical smoke-filled rooms as the police and a collection of criminal bosses (lead by a sneering, swaggering master criminal in a black coat who's obviously a stand-in for a SS officer) each try to figure out how to find the killer. Then when the criminals get a line on the killer the film becomes the world's most insane caper movie as Lorre is cornered in an office building after hours as an army of burglars and thugs methodically take the place apart to find him. Finally it's a psychodrama as Lorre is put on "trial" and defends his actions as a compulsion he has no control over.
It's felt that the film was Lang's reaction to the forces taking over Germany at the time (it was made in 1931). Berlin is dark and dank, everyone is equally unattractive, and aside from a few grieving parents no one really gets our sympathy. And Lang doesn't go for easy answers either. Lorre is a monster, but if he is insane should he be condemed for it? Plenty of people who supposedly can tell right from wrong in the film accuse neighbours and colleagues with no evidence and form an instant mob to assault a man who was asked the time by a child.
Nearly 80 years old and it leaves most modern films lying in the dust. That's Genius and Art.