The young Belarusian historian Mieczysław unexpectedly finds a valuable item - a silver capsule from the time of the last Polish king and Grand Duke of Lithuania Stanisław August. During his captivity in one of his castles, the king hid the artifact, placing a note with a riddle in it, which Mieczyslaw, together with his friends, Vika and Vladimir, undertakes to solve. They manage to find out that the monarch was a member of the Freemasons organization, but at this time strangers begin to interfere in the investigation... The book is based on real facts. All main characters have real prototypes.