Private Investigator Nora Simeon and her partner Eyre work for the Commission, the secret organization that regulates the summoning of demons in the financial industry. It's the Commission's job to suppress all public knowledge of sorcery. So when a mysterious letter from a person who calls themselves l'Invocateur threatens to expose the secret to the world, it's Nora's job to track him down.

In search of l'Invocateur, Nora and Eyre are off to Paris, but before they go, Madame Villiers, Nora's mother and one of the Commission leaders in New York, reveals that Nora's father may be in danger.

Nora has never known her father, a man named Simeon who disappeared when she was an infant. Spurred by her mother's warning, Nora hunts both her father and l'Invocateur in France. From the elegant halls of the Société Générale to the altar of the Sacré-Coeur, Nora and Eyre find the threads of their search coming together in The Demons of Montmartre.