Sergiusz Piasecki, the author of several widely read thrillers including the books about his own spy activities, became an icon of a Polish spy agent in the interwar period on the East. Nowadays, having an access to the recently declassified materials of Polish intelligence services stored in the Russian State Military Archive in Moscow, Sergiusz Piasecki’s real carrier as a spy is worth analysing, especially the final period. In August 1922, he was enlisted to work as a spy. Piasecki was obliged to gather information about the Bolshevik army located in Słuck. His work was finished in December 1923. The three presented documents are the final stage of Sergiusz Piasecki’s activity as a spy.