By order of his superiors, London police inspector Thomas Pitt is investigating a murder and robbery case that dates back three years. In the library of a respectable mansion located in Hanover Close, diplomat Robert York was killed with a blow to the head at night. Thomas soon encounters many strange things. The crime was clearly not committed by a professional, and the stolen items and papers never surfaced anywhere. In addition, before the murder, a mysterious woman in a purple dress, whom no one knew, was seen several times in the house. Having learned about another criminal mystery, Thomas's wife Charlotte and her sister Emily immediately begin their own investigation. But things turn out in such a way that Lady Emily Ashward has to go as a servant to a rich house, and Thomas Pitt goes to prison on charges of murder...