Winter 1826. An investigation into the Decembrist uprising is underway in the Peter and Paul Fortress. Meanwhile, mysterious murders are taking place in St. Petersburg. Moscow detective Lavr Petrovich Perekhodov is trying to understand the motives of the criminal. All this looks like revenge, because those killed were informers against the Decembrists. On the body of each victim, Lavr Petrovich also finds pieces of paper with poetic lines. Who will be the next victim of the criminal? The nobleman Alexander Boshnyak, released from investigation, joins the investigation and becomes the victim of an assassination attempt as a secret agent embedded in the ranks of the Decembrists.