Paleographer and writer Anton Glebovich Kosmich finds in the binding of an ancient book a parchment with encrypted information about treasures hidden in the dungeon of the castle. It gradually becomes clear that the last scion of the family of the Olshansky princes, who collaborated with the Nazis during the war, is also looking for the treasure. In the dungeon of the castle, along with treasures of the 17th century, materials from the SD archive are also hidden. The spaceman is in mortal danger.