Child murderer Hans Beckert (Lorre) walks the streets of Berlin, all the time whistling a creepy tune as he abducts and kills small children.

The police are largely unable to find any leads, so an underground crime circuit launches its own search for the dastardly perpetrator.

Clearly one of the earliest documentary-style explorations of a serial killer, M is an expressionistic masterpiece in a class all its own.

Director Lang, who crafted the silent sci-fi classic Metropolis (1927), creates a mood with M that's supremely unsettling, foreboding.

Lorre is spectacular as the pudgy and unassuming Beckert, in a performance that couldn't fit him more perfectly.

The 'M on coat' chase through the city streets is one of the film's great highlights, but it's the 'vigilante justice' sequence at the climax that stands out as the most stirring moment in M.

Also known as The Murderers are Among Us.

German: Eine Stadt sucht einen Mörder.