The Sleeping Partner heralds the return of the serene and self-reliant Miss Sarah Tolerance, Fallen Woman and Agent of Inquiry. This time, Miss Tolerance is seeking not a missing trinket or the solution to a crime, but a living person - a young gentlewoman who has vanished from under her wealthy family’s roof, apparently bent on a scandalous elopement with a mysterious man whom nobody knows. Her elder sister is desperate to find and forgive her - and Sarah, haunted by the similarities between this girl’s story and her own, is determined to assist - but with nothing to go on but a false name and a small sketched portrait, the case is proving her most difficult yet.

As the search intensifies, Miss Tolerance must adversaries who will stop at nothing to keep her from finding the girl. And, more than she had ever expected, the mystery surrounding this young woman so like and yet so unlike herself forces Miss Tolerance to confront the voices of her own past, and to question what her future may hold.