The butchered bodies of a young woman and her maid are discovered in a train-carriage. The dreadful slashes to their stomachs are rendered even more awful by the bizarre presence of scorpions, writhing inside the wounds.

Assigned to the case, Criminal Counsellor Eberhard Mock and his assistant Herbert Anwaldt must search for the truth within a society in the malevolent grip of the Gestapo, where corrupt ministers torture Jewish merchants for false confessions and Freemasons protect their secrets with blackmail and violence.

In a city already drenched in fear, Mock and Anwaldt's hunt for the killer leads them to the time of the Crusades, an era when secret sects practised ritual murder.