When Mary Quinn is ordered to work undercover after a petty thief steals from Buckingham Palace, she receives attention from a prince, wonders at a murder suspect's shared name with her father, and struggles with her feelings for James Easton.

Rescued from the gallows in 1850s London, young orphan and thief Mary Quinn is offered a place at Miss Scrimshaw's Academy for Girls where she is trained to be part of an all-female investigative unit called The Agency and, at age seventeen, she infiltrates a rich merchant's home in hopes of tracing his missing cargo ships.