Directed by Fritz Lang in 1931, who considered it his own best work. It was also Lang's first sound film, which shows in a particularly odd way: there's no soundtrack music - period, end of story. In fact, there are at least a couple scenes that are totally silent in this Criterion cut of the movie. Cars arrive at a scene, people get out and talk quietly ... but there's no sound at all, which is quite disconcerting.
The movie is set in Berlin, which is haunted by a series of disappearances of young children. The police are desperate to catch the perpetrator, but after most of a year they have few clues. But the police city-wide crack-down has made business for the criminal classes much more difficult, so some of the city's leading criminals get together to try to figure out what to do about it. This leads to the movie's most implausible idea, which was that the beggars of the city are fantastically well organized and could all be used and relied upon as a spy network.
Peter Lorre stars as the murderer, in the movie that brought him to fame. The grand finale shows him ranting in a way that I think was taken quite seriously at the time, but kind of smells like scenery-chewing these days. Not to mention the absurdity of the whole criminal courtroom set-up: we had at this point devolved into allegory rather than dramatic film. Although it does poke at an interesting question: what do you do with someone who has a compulsion to abduct and murder children? If they're "not responsible" by way of insanity, they get treatment and then are released and often commit more crimes. Or do you kill them even though they're legally not responsible, thus saving society from the threat? Either way, the lives lost aren't restored.
I've written some unkind things about the movie and I'm not sure it deserves the "classic" title it gets, but it's well done and interesting.