This fourth book by this very gripping author opens with Mrs Harriet Westerman and her companion, the reclusive amateur anatomist Gabriel Crowther being called, in March 1784, to the Duchy of Maulberg, Holy Roman Empire of the German Nation. During a nobility dance at a masked ball in a small market town, the beautiful Lady Martesen is murdered. The Englishman Daniel Clode is found by her body, his wrists slit and his memories blurred and nightmarish. What has he done?

It seems that Daniel and his new wife Rachel, the younger sister of Harriet Westerman, were visiting the Duchy following their marriage when this calamity occurred. During the festivities Daniel put on a carnival mask which appears to have been drugged and the chemicals penetrated his body via his face and caused him to lose consciousness and have such nightmarish dreams. The victim appears to have been smothered. He is taken into custody.

Because modes of travel at that time were primitive it takes Harriet Westerman and her party over a month of tiring, tedious travel to get there to the Duchy of Maulberg if they are to save Daniel from the executioner's axe.

Gabriel Crowther is the wealthy, close friend and neighbour of Mrs Westerman and they live near each other in Sussex. Crowther is entitled to call himself 'The Baron of Keswick', as he inherited the title after the tragic murder of his father in 1751. His older brother, the normal heir to the title was accused, tried and convicted of the murder of his father and hanged. However, Crowther decided to ignore his destiny and, calling himself Gabriel Crowther instead, sold his father's property and invested the money. He then spent several years studying in Europe at a German university to become an anatomist, which is the eighteenth-century equivalent of a medical examiner or forensic anthropologist, and as recounted in the three earlier books he uses his skills, with the help of Mrs Harriet Westerman, in solving murders.

When Crowther and Mrs Westerman arrive in Maulberg in May 1784, they settle at their accommodation and then make some attempts to discover what motivated the murder of Lady Martesen. Whilst this was going on her one of her associates, Mr Michaels was doing his own investigation, secure in the knowledge that as he was locally born he was able to converse in the common dialect and did not have the linguistic problems of Harriet and Crowther who were only able to use German, French or English to speak with the natives. He learnt a lot, through speaking to many of the citizens that was very useful to the investigation.

In this multi-faceted story, between the chapters describing the difficulties of Mrs Harriet Westerman and Gabriel Crowther in investigating the murder is another story told about a Jacob Pegel who befriends Lord Florian zu Frensel after he attends a maths class at the University and they then got drunk at a local tavern and go back to Pegel's apartment for the night; Pegel is interested in learning about Freemasonry and other secret societies from the Lord.

It seems there is another person in Maulberg that Harriet would rather avoid and whom she has not seen for some years and he is called Manzerotti. He is a eunuch who sings counter-tenor opera arias in concert and Harriet believes he was responsible for the death of her late husband.