[1] It is the fourth novel in a series featuring her amateur detective, the politician Scott Egerton.

[3] It takes the form of a locked room mystery, a popular branch of the genre during the Golden Age of Detective Fiction.

By coincidence, Egerton is at a disreputable inn in Paris when the proprietor is visited by the wealthy Sir Henry Archer, owner of the famous Archer Library, carrying several valuable books with him.

Obviously in a state of terror, he demands that his room be locked and his window barred.

Nonetheless the following morning he is found in bed, stabbed to death.