Penniless and stranded in France after a bid to escape her cruel uncle goes awry, Cornelia Shaw is far from the Parisian life of leisure she imagined. Desperate and lacking options, she allows herself to be recruited to Napoleon's Grande Armee. As a naturalist, her near-magical ability to heal any wound with herbal mixtures invites awe amongst the soldiers...and suspicion. For behind Cornelia's vast knowledge of the natural world is a secret she keeps hidden - the flowers speak to her through a mysterious connection she has felt since childhood. One that her mother taught her to heed, before she disappeared.

Then, as Napoleon's army descends on Waterloo, the flowers sing to her of a startling revelation: a girl who bears a striking resemblance to Cornelia. A girl she almost remembers - her sister, lost long ago, who seems to share the same gifts. Determined to reunite with Lijsbeth despite being on opposite sides of the war, Cornelia is drawn into a whirlwind of betrayal, secrets, and lies. Brought together by fate and magic at the peak of the war, the sisters try to uncover the key to the source of the power that connects them as accusations of witchcraft swirl and threaten to destroy the very lives they've fought for.