Czechoslovakia, 1935: Viktor Kosárek, a newly trained psychiatrist who studied under Carl Jung, arrives at the infamous Hrad Orlu Asylum for the Criminally Insane. The facility is located in a medieval mountaintop castle surrounded by forests, on a site that is well known for concealing dark secrets going back many centuries. The asylum houses six inmates—the country's most treacherous killers—known to the terrified public as the Devil's Six. Viktor intends to use a new medical technique to prove that these patients share a common archetype of evil, a phenomenon he calls The Devil Aspect. Yet as he begins to learn the stunning secrets of these patients, he must face the unnerving possibility that these six may share a darker truth. Meanwhile, in Prague, fear grips the city as a phantom serial killer emerges in the dark alleys. Police investigator Lukas Smolak, desperate to locate the culprit (a copycat of Jack the Ripper), turns to Viktor and the doctors at Hrad Orlu for their expertise with the psychotic criminal mind. And Viktor finds himself wrapped up in a case more terrifying than he could have ever imagined.