Johan is a cold and calculating killer with a mysterious past, and the brilliant Dr. Kenzo Tenma is the only one who can stop him! Satanic conspiracies and serial killings pave the way for an exciting, intricately woven plot in this thriller masterpiece.

Tenma learns that ten years ago, a woman lived in an old apartment building in the center of Prague with twin children, above a sign with three frogs. The neighbors also tell him about the Czech secret police and a suspicious fire. Could the twins be Johan and Anna? Meanwhile, Suk, a young detective in the Prague police, meets a beautiful young woman, Anna, in a bar. Suk also crosses paths with Tenma as a new terrifying situation threatens them. The BKA director, Lunge, is now convinced of Johan's involvement in the serial killings and comes to Prague to find out what Johan really is. He learns that an agent of the Czech secret police created the strange illustrated book The Unknown Monster, which seems to have deeply affected Johan in his childhood. Lunge manages to locate the Red Wheel, the inspiration for the book's author. There, behind a thick layer of plaster, he discovers an old door and the real nightmares that lie behind it!