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.