A London lady who's skilled in the practice of mesmerism, Lavinia Lake has launched a new career as a private investigator, a startling and slightly scandalous occupation for a woman in Regency England. Strong-willed and independent, Lavinia isn't a typical society widow, a fact which alternately attracts and frustrates her enigmatic partner, Tobias March. Lavinia and Tobias have a prickly and passionate relationship, and Tobias is determined to protect her, while Lavinia is just as determined to be an equal partner in their business.
When the wife of mesmerist Dr. Howard Hudson is murdered and a rare antique bracelet known as the Blue Medusa disappears, the doctor seeks Lavinia's help. Tobias is reluctant to become involved in the matter but Lavinia feels compelled to help her late father's associate. Their attempt to uncover the murderer leads them to obscure antiquities dealers, secret organizations, amoral heirs, and ladies of the evening. And when a madman threatens Lavinia's life, only Tobias can hope to save her.