In the first film of the trilogy, a famous knight is murdered in the castle having just arrived from Gotland; the troops he had commanded had defeated and executed the pirates that had terrorized Hansa merchant ships. The killer has stolen the victim’s gold chain, chopped off his head and filled his mouth with coins. Melchior discovers that the murdered knight was after the mysterious Prisoner of Tallinn however, for some reason, he wasn’t looking for him in the jailhouse, but in the Dominican monastery. The next day, the old wall of the monastery collapses and a shocking series of murders erupts the master mason’s head is chopped off and his mouth is filled with coins in a copycat manner, and two monks are poisoned with arsenic. Is the same murderer behind all these killings or is someone imitating a crime to cover his tracks? Melchior realizes that the key to the mystery lies in the question who or what is the mysterious Prisoner of Tallinn?