They had a secret. They had to die. On a Sunday, a body is found in the residential building of a decommissioned factory. It is the former operator of the plant, Theodor Reifenrath, as Chief Detective Pia Sander determines. In a dog kennel, she and her boss Oliver von Bodenstein make a gruesome discovery: next to a nearly starved dog, human bones are scattered, and the forensic team uncovers more and more horrifying details. Reifenrath had lived very reclusively since his wife Rita took her own life twenty years ago. No one in the village wants to believe that he was a serial killer. Forensic expert Henning Kirchhoff can identify some of the victims who were murdered years ago. All were women. All disappeared on a Sunday in May. Pia is convinced: the murderer is still at large. He is looking for his next victim. And soon it will be early May.