The book follows Lucas Corso, a book detective who is hired to authenticate a rare manuscript by Alexandre Dumas. As he delves into the investigation, he finds himself entangled in a mystery involving two other books: a rare edition of "The Three Musketeers" and a satanic text, "The Book of Nine Doors of the Kingdom of Shadows". The narrative becomes a complex puzzle as Corso uncovers connections between the books, while also dealing with a seductive woman who seems to have stepped out of a Dumas novel, a mysterious killer, and a group of bibliophiles obsessed with Dumas. The lines between fiction and reality blur as he gets closer to the truth.

The novel explores the relationship between two remarkable women: Blanche Wittman, a patient at the Salpêtrière hospital in Paris who became a renowned medium, and Marie Curie, a physicist and chemist who conducted pioneering research on radioactivity. The book delves into their friendship, their scientific collaborations, and the profound impact they had on each other's lives. It also explores the broader themes of scientific discovery, the nature of genius, and the role of women in a male-dominated society.