In a city of light and reason, a killer's weapon is a work of art. Her mind is the only thing that can stop it.

Paris, 1750. Céleste Vaudry is a ghost. By day, she is a reclusive automaton-maker, hiding her genius from the guilds that destroyed her family. By night, she creates breathtaking clockwork wonders for a secret clientele, her only solace found in the predictable logic of gears and springs.

Bastien de la Roche is a man haunted by his past. A former musketeer turned private investigator, he trusts only cold, hard evidence. When a series of impossible murders terrifies the aristocracy-each victim found in a locked room, the only clue a single, mechanical feather-Bastien's investigation leads him to Céleste's shuttered workshop.

He is convinced she is either the killer or the key. Forced into a reluctant alliance, Céleste must use her unparalleled knowledge of mechanics to unravel the secrets of a weapon that defies logic, while Bastien must navigate a conspiracy that reaches the highest levels of the French court.

As they are drawn deeper into a world of gilded secrets and vengeful artisans, their fiery intellectual rivalry ignites into a dangerous attraction. But in a city where lies are currency and trust is a fatal mistake, they must learn to have faith in each other. With a killer's masterpiece set to perform at a royal masquerade, Céleste and Bastien have one chance to stop a symphony of death, or become the final notes in the clockwork nightingale's song.