Based on Andreas Gruber's bestseller: A female corpse is found in Munich's Frauenkirche, suffocated by a large amount of ink—a reference to the "bad guys" in "Struwwelpeter." Munich detective Sabine Nemez takes over the investigation. However, parallels to cases in Leipzig and Cologne bring the Federal Criminal Police Office (BKA) into play, where the stubborn but brilliant case analyst and forensic psychologist Maarten S. Sneijder is investigating the series of murders. Reluctantly, but convinced by Sabine's instinct and talent, Sneijder allows the young detective, for whom solving the case is a personal concern, to continue her involvement in the investigation. The bloody trail leads the quick-witted duo to Vienna, where psychologist Helene Berger has received a package containing a severed thumb—the story of the "thumb-sucker." This makes her part of a bizarre psychological game, for every murder is preceded by a riddle from the murderer...