As Europe prepares itself for a calamitous war, six homicidal lunatics - the so-called 'Devil's Six' - are confined in a remote castle asylum in rural Czechoslovakia. Each patient has their own dark story to tell and Dr Viktor Kosarek, a young psychiatrist using revolutionary techniques, is tasked with unlocking their murderous secrets. At the same time, a terrifying killer known as 'Leather Apron' is butchering victims across Prague.