The titular adviser Erast Fandorin takes part in hostilities during the Russian-Turkish war of 1877-1878. As a Serbian volunteer, Erast Petrovich meets a charming young lady - Varvara Andreevna Suvorova, who is heading to the location of the Russian troops to see her bridegroom. With her help, Fandorin manages to unravel the complex and mysterious case of espionage.