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 book revolves around a Swiss classics teacher who, upon encountering a mysterious book by a Portuguese author, becomes obsessed with the life and thoughts of the writer. This obsession leads him to abandon his job and life in Bern, embarking on an impulsive journey to Lisbon. In his quest to understand the author's fate and the resistance movement against the Portuguese dictatorship that the author was involved in, the protagonist explores themes of history, philosophy, and the complexities of human character. As he delves deeper into the past, he encounters various individuals who were connected to the author, piecing together a narrative that challenges his own understanding of life and identity.