Angelo Stassner is planning a new coup: blood herb paintings. But now the gallery owner lies in his own blood. He has been stabbed. Is there a connection between the murder and the blood herb paintings? The police ask herbal expert and amateur detective Father Gwendal for help. He puzzles over a strange symbol left behind by the deceased. The traditional knowledge of the power of plants ultimately leads Gwendal to the astonishing solution of the case.