Von Harbou co-wrote the screenplay for M (1931), a landmark early sound-era thriller about a child murderer, directed by Fritz Lang.
M, directed by Fritz Lang and co-written with Thea von Harbou, premiered in Berlin on May 1, 1931. The film, one of the earliest and most acclaimed sound films in German cinema, told the story of a serial child murderer and the parallel manhunts conducted by police and the criminal underworld of Berlin.
Von Harbou's screenplay was notable for its psychologically complex treatment of criminality and social panic, moving away from the more expressionistic and allegorical style of earlier collaborations like Metropolis toward a starkly realistic, almost documentary tone suited to the new medium of synchronized sound. The film is widely regarded as a precursor to the film noir genre and one of the finest achievements of Weimar-era cinema.
M showcased von Harbou's versatility as a screenwriter capable of adapting her craft to new technologies and narrative styles, further establishing her reputation as one of the most important screenwriters of her generation. Her ability to construct tense, socially resonant narratives contributed enormously to the film's critical success and enduring legacy; it continues to be studied in film schools worldwide as a masterclass in suspense and social commentary.