The son of a top manager lies dead in the swimming pool of the elite boarding school at Hamberg Castle. At first glance it looks like suicide. Klara Blum believes that too - until she first doubts. When Kai Perlmann finds a torn laundry button in the pool's filter system and Klara discovers the matching duvet cover, it becomes clear that the boy was drowned like a cat in a sack. The slippery arrogance of the classmates causes the commissioner and her assistants not to let anyone in the school notice that they are investigating an extraordinarily cold-blooded violent crime. A game of cat and mouse with the suspect students begins. Despite Klara's psychologically cautious tactics, they soon realize that their "perfect" murder may be exposed. Since they also have to cover up a large-scale financial fraud with which the deceased in the swimming pool tried to blackmail them, and an accomplice begins to lose her nerve, they are already thinking about the next step. They are so cold-blooded that Klara seems to fall behind in their investigations. But when a classmate gossips during interrogation, Klara Blum and Kai Perlmann quickly catch up. It will be a race against death.
—ARD Das Erste