Because a witness failed to appear at the police station despite being summoned, Hubert and Girwidz set out to find the man. Too late—he's found dead in the settling tank, his workplace. Ulrike Mettmann, the head of the wastewater treatment plant, assumes it was a tragic accident. But it quickly becomes clear: someone intervened. When it also turns out that the tap water is contaminated, a state of emergency is declared in Wolfratshausen. For Hubert, however, this warning comes too late. He has already drunk some of the contaminated water… Sabine Kaiser takes the ailing Hubert off the case. While the residents of Wolfratshausen hoard water supplies and Riedl is tasked with ensuring the drinking water supply in the precinct, the chief takes over the investigation alongside Girwidz.