London, 1716. Touring the gallery of the dome of St. Paul's Cathedral, Hester Kean hears a mysterious whispered threat. When Lord Wragby is murdered, she realizes that the threat was directed at him and must have come from one of the gentlemen she saw in the gallery that day. Lord Wragby's father, the powerful and ruthless Marquess of Ireton, blames Hester's friend James Henry for the killing. Hester must solve the murder to save him, but it is not until the outlawed Viscount St. Mars, aka Blue Satan, returns to claim her hand that she has the means to bring the killer to justice.