The childless couple, the writer Andrei and the artist Olga, adopt eight-year-old Ksyusha. The director of the orphanage where the girl was brought up is Andrei's ex-girlfriend Irena. She still passionately loves Andrei and intends to return him at any cost, even if this will require destroying his family happiness. Meanwhile, Andrei is composing a fantasy novel about the dark side of St. Petersburg. On the pages of his book, the city on the Neva turns into a phantasmagoric world, where time itself has stopped, and the streets are inhabited by soulless creatures. Suddenly Ksyusha finds herself in the world of "dark Petersburg" from Andrey's novel, where she meets a mysterious friend Paramon. Many dangerous adventures await the heroes...