A young woman is found dead in a lake that is said in legend to be inhabited by ghosts who drag people into the abyss. Everything points to suicide, but agent Maris Bächle and her new colleague Konrad Diener from the Freudenstadt homicide department have their doubts. When a second woman drowns in the lake, the inspectors soon find a link between the two cases. A trail leads them to Hans Katrein, emeritus professor of theology, who has recently returned to his birthplace in the Black Forest after years abroad.