While studying the economic history of Europe, French professor Jean d'Hayon became interested in political history and, unexpectedly becoming a writer, began publishing novel after novel about the era of the Three Musketeers. To this turbulent period, to this day full of unsolved mysteries, he dedicated his famous series of historical detective stories about Louis Fronsac. October 1643. The Thirty Years' War is coming to an end. Diplomats from France and other countries are preparing for the Munster Conference, where the fate of Europe will be decided. And suddenly it turns out that in the most secret department of the Ministry of Foreign Affairs - in the encryption bureau - there is a spy, and the contents of the encrypted dispatches become known to the enemy. Cardinal Mazarin is confident that only Fronsac can find the traitor. The matter turns out to be extremely dangerous. Louis has to fight both charming schemers and ruthless villains. In addition, it is urgent to create a new code that the enemy cannot decipher. And in this Fronsac should be helped by the best minds of France - mathematicians Blaise Pascal and Pierre Fermat.