Fritz Lang was interested in the case of the Dusseldorf child killer, Kurten, and prevailed upon his wife, Thea von Harbou, to help him write a screenplay on the man. The original title for the film was Morderer unter uns (Murderer among us), but this title presented many problems at the time of production (1930); the rising Hitler regime mistook the title as an attack upon them, and Lang was refused entry to the studio until they discovered the true nature of the film.

It was the director's first sound film and his undoubted masterpiece. The life and death struggle between anarchy and authority is concentrated in a single character, the psychopathic child-murderer, caught helplessly in the grip of impulse and hallucination. The slory tells of a town terrorised by this maniac, and the failure of the police to catch him until the criminal underworld, alarmed by continual police raids, take their own steps and capture him. The film's climax is the wretched creature's "kangaroo court" trial by the assembled criminals and beggars of the city, and his hideous, grovelling, impassioned confession, before being rescued "in the name of the Law and the People".

No subject could be more open to facile, sensational treatment, yet Lang handled it with tact. The pace is slow, and the imaginalive use of sound is unparalleled, but the film's real triumph is the faultless characterisation of the murderer, incomparably portrayed by Peter Lorre as a fat, timid, infantile little man, who eats apples in the street and pulls melodramatic faces in the mirror in the privacy of his drab lodgings.