Conrad Ketteler was about to take office as environmental advisor to the new member of the state parliament, Günter Balried. But the biologist and conservationist Ketteler is found dead in Lake Constance. It was at the center of a dispute between conservationists and a group of commercial fishermen fighting the protected but fish-predating cormorants. During their investigations, Klara Blum and Perlmann discover that Ketteler had secretly collected evidence of illegal activities by the fishermen. He was either murdered because he stood in the way of the fishermen with his principles, or because he was too private in his private lifehad few principles. A witness testifies that she saw Fischer Gasser in Ketteler's house on the evening of the crime. Gasser vehemently denies this. Klara is convinced that Gasser is lying, but cannot prove it to him.
It is also questionable whether he is lying because he murdered Ketteler - or rather to protect Günther Balried. Although Balried is the protagonist of a green party, he seems to have more in common with Gasser than he cares to admit. Klara begins to concern herself with Balried's past. It turns out that Conrad Ketter didn't die in the lake at all, but in the water of the fish hatchery...