The film is loosely based on "William Wilson", a short story by Edgar Allan Poe, the poem "The December Night" by Alfred de Musset, and Faust. The Faust elements are obvious, the other two less so (though the Musset quotation might give it away). This is really a great early example of horror literature on screen.
Cinematographer Guido Seeber utilized groundbreaking camera tricks to create the effect of the Doppelgänger (mirror double), producing a seamless double exposure. Hanns Heinz Ewers was a noted writer of horror and fantasy stories whose involvement with the screenplay lent a much needed air of respectability to the fledgling art form.