The engineer Claudio Messina dies on a construction site. Marie Brand quickly finds evidence pointing to a murder that was supposed to be disguised as an accident. The behavior of the foreman Giuseppe Costa at the scene of the crime raises the suspicion that the company employs undeclared workers. When the police file of the murdered man is still secret, the case becomes mysterious. Was the victim in the witness protection program? A member of the mafia? Marie Brand and Jürgen Simmel discover that Claudio Messina was hot on the heels of senior contractor Schimkat, who was cooperating with a front company in Italy to procure undeclared workers.
When the inspectors look for the company founders, it turns out that they are two Sicilian goatherds who are staying in a men's hostel in Cologne while the dubious contracts are being signed. The mastermind behind this business relationship is the young Italian Carla. While the inspectors are on their trail, another person dies: the foreman Giuseppe Costa is found shot dead. Next to his corpse is the identification mark of a mafia family known to the police.