A well-know bibliophile is found hanged days after selling a rare manuscript of Alexander Dumas's classic, The Three Musketeers. Across Madrid, Spain's wealthiest book dealer has finally laid his hands on a 17th-century manual for summoning the devil. Lucas Corso, solitary and obsessive, is the detective hired to authenticate both texts. But the further he follows the trail of devil worship, the more it leads him back to Dumas. He's the unwitting protagonist in someone's evil plot, but is he sleuth or hero, Sherlock Holmes or d'Artagnan?