By Georgette Heyer
The story is summarized by the author herself towards the end of the novel:
'"Once upon a time—there were two brothers. I have forgotten their names, but since they detested each other, I will call them Cain and—er—Abel ... The hatred grew and flourished until I believe there was nothing one brother would not do to spite the other."
"Cain, being the elder of these two brothers, succeeded in due course to his father ... Cain's succession but added fuel to the fire of hatred, and whereas our friend Abel was consumed of a desire to stand in his brother's shoes, Cain was consumed of a like desire to keep him out of them ... Cain took a wife unto himself and doubtless thought himself secure ... But ... the years went by, and still there came no son to gladden Cain's heart ... Abel ... grew more and more jubilant, and I fear he did not hesitate to make—er—a jest of his brother's ill-luck. [Cain's wife] raised her husband's hope once more. This time Cain determined that there should be no mistake. When madame's time was upon her, he carried her off to his estates, where she was delivered of—a daughter." ... Now observe the cunning of Cain. On his estate ... there dwelt a farm-labourer ... whose wife had just presented him with a second son ... [Cain] bribed this peasant to give him his lusty son in exchange for his daughter."
"Cain presently brought his family back to Paris ... leaving instructions that his daughter's foster-father was to leave his estate for some remote spot ... For twelve years [Cain's daughter] remained in the heart of the country, with her foster-parents, and was reared as their own child ... A plague struck down both foster-father and mother, but my heroine escaped, as did also her foster-brother ... She ... was taken to Paris by her foster-brother, a youth many years her senior ... He bought a tavern in one of the meanest and most noisome of your [Paris] streets. And since it was inconvenient for him to have a girl of my heroine's tender years upon his hands, he dressed her as a boy ... I shall not discompose you by telling you of her life in this guise ... Further, he married a slut whose care was to ill-use my heroine in every conceivable way. At this woman's hands she suffered for seven long years ... During those years she learned to know Vice, to Fear, and to know the meaning of that ugly word Hunger. I do not know how she survived."
"Then ... Fate stepped in again, and cast my heroine across the path of a man who had never had cause to love our friend Cain. Into this man's life came my heroine. He was struck by her likeness to Cain, and of impulse he bought her from her foster-brother. He had waited for many years to pay in full a debt he owed Cain; in this child he saw a possible means to do so..."'
So there you have it, pretty much the whole plot of the story. The girl, Leonie, is found by the Duke of Avon, an English lord visiting Paris. Her unusual coloring, bright red hair and striking black eyebrows and lashes grabs his attention. At first, the Duke believes she is a by-blow of his enemy, the Comte de Saint-Vire, a French nobleman that the Duke had a run-in with years ago. Comte too has bright red hear and striking black eyebrows. Doesn't take the Duke long to figure out that the person he thought was a teenage boy is in fact a young woman. Being an intelligent man, he realizes that Leonie was given up by the Comte as described above. This knowledge is the weapon the Duke will use to bring the Comte to his doom. And in the process the Duke will lose his heart to a beautiful, wild young woman who is twenty years younger than he is.
This is a very interesting tale. In the first part Leonie serves the Duke as a page, but eventually he sends her to Britain to learn to be a young noblewoman. Then he brings her back to Paris and introduces her to Paris high society as his adopted daughter. Let the fun begin!
Wikipedia has an entry about These Old Shades.