In medieval Tallinn, the famous knight of the Order who freed the Baltic Sea from pirates is murdered. The hero's head is chopped off and his mouth is stuffed with coins. The gold chain he bought the same day is missing. The Sheriff orders Melchior the Apothecary, who can talk to the dead, to investigate the crime. The clever young man discovers that the murdered knight was looking for a mysterious "Prisoner of Tallinn" and clues lead to the Dominican monastery. A series of terrible crimes is unleashed - anyone who comes into contact with this secret is in danger. At the same time, the apothecary has doubts about his apprentice, who seems to be hiding something...