Endless thanks to my literary agent, Jill Marr, who plucked me from a Twitter pitch party to get things started. I could not have journeyed this dazzling road without you, truly. Your spark, insight, and good humor have all been more than I hoped for. You are the very best.

Thank you to Megha Parekh, who believed in this book from the get-go and not only understood it but valued its unique set of characters. I am so appreciative of you as a guide and editor, to have your brains and vision in my corner; you knew exactly what my book needed to level up. To Caitlin Alexander, who lit my world on fire with her enviable judgment, thank you for reading and rereading, and rereading, still. I could not have asked for a better developmental editor to cut out subplots and make these sisters shine. To Sarah Shaw and the entire crew at Thomas & Mercer, thank you for making my dream a reality.

Special gratitude to Arielle Max Drisko, who convinced me that writing a full-length novel was possible—a thought I carried with me for two more years before I actually attempted the feat (despite many journals and rambling diary entries from childhood). Additional callouts must be made to writer and editor Nicole Tone, for taking my story and nudging it along when my manuscript was still in its early form.

To my friends in France who puzzled at me when I first said I was writing a book (“—de . . . quoi?”) and yet let me mine their histories a bit more, thank you for smiling and serving more Calvados. With express recognition to the city of Paris, the Paris catacombs, and the six million-plus souls that reside belowground, thank you for serving as inspiration for the setting of this story, and giving me some of the best experiences of my life. A bientôt, j’espère!

This book couldn’t have been written without the expertise of Dr. Erin Healy, my genius friend and generous resource, even as I asked question after question regarding things clearly not scientifically possible. I appreciate you. Any error in this book regarding the practice of medicine, the subject of science as a whole or otherwise, is my fault alone (or that of my short-lived biology major).

To my writing friends and critique partners who read this story in its first, intermediate, and final iterations—Heather Lettere, Elaine Roth, Raimey Gallant, and Cathy Holst—thank you for the quick reads and gut checks. In unique ways, you each kept me going with your camaraderie, talent, and incisive commentary. Thank you a thousand times over.

To my in-laws, your support of this whole writing thing, since day one, has been striking and thoughtful. I completely lucked out with you all.