The 1930s. A Frenchman appears in the estate of Iwon Gąsowski near Vilnius, who necessarily wants to buy an indebted property and a house falling into decay. The owner does not want to sell it, then the old door disappears from the house.
First one, then the other ... A high school professor, Piotr Gąsowski, a historian, asks his brightest student, known for his detective skills, Adam Cisowski, for help in solving the mystery of incomprehensible thefts in his brother's house. And everything should be attributed to the letter of Kamil de Berier, a colonel in the Napoleonic army, found in Paris in 1936, who left clues about the hidden treasure during his retreat from Moscow, shortly before his death.