The book is told from the point of view of Kate's (fictional) 11-year-old niece, Nell, who arrives on the Chicago doorstep of her last surviving relative lugging a "heavy sack of sorrows." If her Aunt Kate rejects her, it's the miserable Home for the Friendless. Luckily, canny Nell makes herself indispensable to Aunt Kate...and not just by helping out with household chores. Kate works as an undercover detective for the Pinkerton Agency, and Nell has an affinity for the kind of close listening and bold action that made Pinkerton detectives famous in Civil War-era America. Aunt Kate's work even brings both of them to Baltimore, at the very moment of an assassination plot against President-elect Abraham Lincoln. With huge, nation-changing events simmering in the background, Nell uses skills new and old to uncover truths about her past (including her family's involvement with the Underground Railroad), and solve mysteries in the present.