Inspector Casstorff and his team are confronted this time with a case that is unusual and frightening: the corpse of 14-year-old Ronja is the starting point for their investigations, which lead to Hamburg's social hotspots. The suspects are as young as the victim: a clique of underage girls who have already made a name for themselves through various acts of violence. Girls who come from extremely disturbed family and social backgrounds, for whom brutality and violence are omnipresent even as children. They know that they are not yet of criminal age - in this respect the interrogations with which Casstorff and his colleague Holicek initially try to shed light on Ronja's murder are hardly effective at all.
Only Jenny Graf manages to gain the trust of one of the suspected girls. Fatma, a young Turkish woman, drives Casstorff insane with her indifferent manner and impudence - until she opens up to him in a quiet moment. The thick mela remains closed; with Lucy, who is caring for her grandfather who has Alzheimer's at home, Jenny notices that she was in love with Ronja. And then there is the quiet and sensitive Marie, Ronja's cousin. The clique really settles in with her because the parents are on vacation without Marie.
Or was Ronja's murderer Piet, who came from a "good family" and finally wanted revenge because the gang of girls regularly "pulled" him on the way to school? With empathy and tireless investigative work, Casstorff, Holicek and Jenny Graf finally succeed in solving Ronja's murder. There is a showdown on board the houseboat on which the young girl lived together with her mother...