Private detective Peter Mang is found half-naked in a construction pit with a dog tattoo on his neck and puncture wounds in his back. Has he become the victim of his secret investigation? The trail leads Inspector Thiel to the villa of the industrialist family Rummel. Despite the clean facade, Sabine and Markus Rummel's marriage obviously seems to be crumbling: after all, the photos taken by the detective show that the factory boss and his new assistant Christine Schauer have more in common than just an employment relationship. Meanwhile, Prof. Boerne is once again investigating on his own. The fact that the murdered private detective looked so similar to him is something that doesn't bother the forensic doctor. The dead man's striking tattoo leads him to a relevant tattoo studio in Münster. Here he meets the Belarusian Jan Sievic – and his fighting dog.