What is this thing coming towards me? Emerging in an era when cinema was still formulating its own conventions, Robert Wiene’s enduring masterpiece (featuring key contributions from Fritz Lang) was not required to fit into any existing template.

In subsequent decades some have sought to define it as the first true horror film (though Georges Méliès's more macabre work surely qualifies) or as the most important cinematic manifestation of expressionism (though critic Kim Newman thinks that a tad "spurious"). More than a few chave seen Caligari, released in 1919, as a grim hangover from the first World War. It is all these things and none of them.

We begin with a framing story in which a man tells of meeting the titular doctor (Werner Krauss) at a carnival. This sinister figure keeps a somnambulist named Cesare (Conrad Veidt, later Hollywood's favourite German) in a box that looks uncomfortably like a coffin. The whey-faced figure appears to be able to predict the future. Or maybe he just bends the future to his own ends. At any rate, the sleepwalker is soon lugging the narrator's female companion about the roofs of an unreal city.

The story is pure hokum and, in its closing stages, compromises with an overly convenient twist. But the atmosphere of the piece has never been equalled. Not yet in thrall to mundane realism, the German film-makers paint in shadows, skew sets to crazy angles and distort perspectives.

One superficial interpretation is that we are looking at the world as seen by a madman. More disturbing (and more convincing) is the notion that the twisted visuals address the unknowable nature of the physical world. The Nazis, of course, labelled Caligari as "degenerate art". There could be no greater compliment.