In a Regency London that isn't quite the one we know, young women of family whose reputations have been ruined are known as the Fallen. Young Sarah Tolerance is one such: a daughter of the nobility who ran away with her brother's fencing-master. Now that the fencing-master has died, everyone expects her to earn her living as a whore.

But Sarah is unwilling. Instead, she invents a new role for herself, and a new vocation: "investigative agent." For Sarah, with her equivocal position in society, is able to float between social layers, unearth secrets, find things that were lost, and lose things too dangerous to be kept. Her stock in trade is her wits, her discretion, and her expertise with the smallsword -- for her fencing-master taught her that as well.

She will need all her skills soon, when she is approached by an agent of the Count Verseillon, for a task that seems routine: reclaim an antique fan he once gave to "a lady with brown eyes." The fan, he tells her, is an heirloom; the lady, his first love. But as Sarah Tolerance unravels the mystery that surrounds the fan, she discovers that she--and the Count--are not the only ones seeking it, and that nothing about this task is what it seems.