In medieval Tallinn, a famous knight who is freed from the pirates of the Baltic Sea is murdered. The hero's head is cut off and his mouth is stuffed with coins. The gold chain he bought the same day is missing. The bailiff orders Melchior, an apothecary, to investigate the crime. The clever young man discovers that the slain was looking for a mysterious "Tallinn prisoner" and the traces lead to the Dominican monastery. A fabulous sequence of bloodwork is unleashed - anyone who comes into contact with this secret is in danger of dying.