On a mild spring day, the body of Ottmar Kinker is found at the Powder Tower in Lindau. The circumstances suggest a crime has occurred. Commissioner Conrad Schielin, who is currently hiking in the hinterland of Lake Constance with his donkey Ronsard, is called back to take over the case. He knows the victim as a reclusive, solitary bachelor who lived with his mother and sister in a house in Reutin and worked as an auditor. Ottmar Kinker led such an unremarkable life that it is difficult to imagine what the motive for the crime could be. Schielin's investigations reveal that the crime did not happen by chance but was carried out deliberately. Furthermore, it turns out that something must have recently changed in Ottmar Kinker's life. The old letters found in the estate also pose a mystery.