The action takes place in St. Petersburg, at the end of the 19th century. A raid was made on the mystical salon "Ariadne's Thread". The owner of the salon, Madame Messmer, is wounded and lies unconscious, her husband is missing. But since the money and jewelry remained untouched, it is clear that this is not a robbery: the raiders were looking for something else... Investigator Polezhaev and his unofficial “assistant” - twelve-year-old daughter Verochka, a smart girl who dreams of being “a detective like dad” - take on the case. The investigation of this case unexpectedly leads our heroes to... an English private detective living in St. Petersburg under the pseudonym Eugene Rainburn. Will our heroes be able to find out that the detective's real name is Sherlock Holmes, and he is hiding in St. Petersburg not from Professor Moriarty, but from someone much more malicious?