How do you find a killer when you're surrounded by madness? 1935. 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 Kosárek, 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. Successfully eluding capture, it would seem his depraved crimes are committed by the Devil himself. Maybe they are... and what links him with the insane inmates of the Castle of the Eagles? Only the Devil knows. And it is up to Viktor to find out.