Nikita and Anna have been married for a long time. Nikita is a well-known detective writer, Anna is a business analyst. They are returning by train from the anniversary celebration of a distant relative. Anna is upset that she was unable to buy tickets for the SV, so they have to travel in a regular compartment car. But their long journey is brightened by their neighbors in the compartment—Alevtina and her little daughter Marusya, who are heading to Moscow. The charming Marusya eagerly chats with her fellow travelers. When they wake up in the morning, the couple goes to the restaurant for breakfast, and when they return, they find the child sleeping alone. The girl’s mother has disappeared…