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.

This novel is set during the Peninsular War and follows the adventures of Richard Sharpe, a British soldier who rises through the ranks due to his bravery and skill in battle. After a personal challenge leads to the capture of a French standard, an eagle, Sharpe must navigate the complexities of military politics, betrayal, and the harsh realities of war. Alongside his loyal men, Sharpe embarks on a daring mission that tests his leadership and combat abilities, aiming to secure a decisive victory for the British army against the backdrop of Spain's rugged landscape. This story combines historical detail with action-packed sequences, showcasing the harshness of war and the valor of those who fight.