An informant in the underworld must hand over materials about the organizers of a series of contract killings among the high society of St. Petersburg to the investigator of the Third Department of the Secret Chancellery of Emperor Nicholas I, Grigory Ivanovich Konvitsyn. However, the detective who arrives at the meeting place discovers the fresh corpse of his agent, Senka Shalogo. It is necessary to catch the killer and intercept the most valuable documents at all costs before he disappears without a trace in the vastness of the capital. But how to catch a criminal, not a single sign of whom is unknown?