Peter Lorre plays Hans Beckert, the main character in Fritz Lang's first talkie. As it's his first leading role in a feature, Lorre can't really be called a "star." The real star is a city full of terrified people. M is about the reactions of Berlin city officials, residents, business owners, and even organized criminals, when a series of mysterious child murders take place.
Anxiety erupts citywide after the disappearance and murder of Elsie Beckmann. Conjecture is rampant: That man followed the little girl up the stairs -- he must be the killer; That man is offering to walk that little girl home -- he must be the killer; this one's a pickpocket -- he must also be the killer.
Unhappy with being lumped in with a child-killer, even the city's criminal element conspires to suss out the killer and administer their own justice, while the police conduct their own investigation. The criminals nab him first, chasing him through Berlin, tracking him down by the chalk "M" slapped onto his shoulder by one of the pursuants. They finally corner him in an office building and spirit him to an underground trial.
There, Beckert delivers a desperate defense. "I can't escape," He cries, "I have to obey it. I have to run, run...endless streets. I want to escape, to get away! And I'm pursued by ghosts. Ghosts of mothers and of those children...they never leave me. They are always there...always, always, always! Except when I do it, when I...then I can't remember anything."
Especially considering when it was released, M is a bold statement about the power of the mind over behavior. The Nazis hated it, calling it "degenerate;" evidence of the moral bankruptcy of the Jews. Years later though, according to IMDb, it was chosen by the Association of German Cinémathèques as the most important German film of all time. Fritz Lang gets the last laugh.