No one looks kindly on the killer of a king.

After unwittingly helping her mother poison King Louis XIV, seventeen-year-old alchemist Mirabelle Monvoisin is forced to see her mother's Shadow Society in a horrifying new light: they're not heroes of the people, as they've always claimed to be, but murderers. She herself included. Mira tries to ease her guilt by brewing helpful curatives, but her hunger tonics and headache remedies cannot right past wrongs or save the dissenters whom her mother vows to purge.

Royal bastard Josse de Bourbon is more kitchen boy than fils de France. But when the Shadow Society assassinates the Sun King and half of the royal court, he must become the prince he was never meant to be in order to save his injured sisters and the petulant dauphin. Forced to hide in the sewers beneath the city, Josse's hope of reclaiming Paris seems impossible―until his path collides with Mirabelle's.

She's a deadly poisoner. He's a bastard prince. They are sworn enemies, yet they form a tenuous pact to unite the commoners and former nobility against the Shadow Society. But can a rebellion built on mistrust ever hope to succeed?