Amateur detectives Augusta and Julia Colebrook use their wits and invisibility as "old maids" to rescue a noblewoman behind enemy lines in this thrilling feminist Regency mystery from New York Times bestselling author Alison Goodman.

Forty-two-year-old Lady Augusta (Gus) Colebrook and her twin sister, Lady Julia, are far from the "useless old maids" Regency high society has deemed them to be. On the contrary, they take advantage of their social invisibility to seek out and protect women and children ignored by society and the law. But their adventures alongside the disgraced Lord Evan Belford have become increasingly dangerous.

When the twins' activities attract the interest of Mr. Whitmore, the head of the government's new intelligence organization, he offers them a deal. Their mission: to retrieve a noblewoman from Napoleon's France. The reward: a full royal pardon for Lord Evan, and with it, the possibility of a life together for him and Gus.

With trepidation, Gus, Julia, Lord Evan, and government agent Mr. Kent head to Paris, the enemy's heartland, and step into the treacherous world of spies, assassins, and bitter police rivalries. As secrets abound and catastrophic tensions rise within the group, so too does the peril from the new Paris police unit, the Sûreté, and the deadly threat from an old enemy. Will they retrieve the noblewoman in time, or will prejudice and distrust tear the group apart and leave them all in the path of destruction?