The city of Schwerin is sweltering under a heat wave never seen before. Detectives Lona Mendt and Frank Elling are on the trail of a serial murderer, who seems to choose his victims at random. But nothing is what it seems.

Detective chief inspectors Lona Mendt (Petra Schmidt-Schaller) and Frank Elling (Sascha Gersak) are investigating a complicated murder case that leads them into a web of dark secrets. In "Dark Heart - The Dead of Marnow 2", the focus of the investigation is twelve-year-old Sarah (Greta Kasalo), a Bulgarian orphan girl. But two weeks after they began their investigation, Lona and Elling are shot. They are in a coma, while Sarah disappears without a trace. What led to this tragic turn of events? And who wanted to silence her? Special investigators Maja Kaminski (Sabrina Amali) and Hagen Dudek (Bernhard Conrad) take on the mysterious case. With deleted files and Dudek's own hidden agenda, their search for the truth becomes a dangerous game. And the girl Sarah is in grave danger of dying. The exciting story unfolds in two parallel narrative strands – a mysterious puzzle that refuses to fit together into a coherent picture.