In New York, the money-driven dealer Dean Corso is a rare-books expert and partner of Bernie, who owns a bookstore. He is contacted by the renowned collector of books about the devil Boris Balkan, who has just acquired the rare The Nine Gates of the Kingdom of Shadows from the collector Andrew Telfer, to verify whether his book is authentic or a forgery. Balkan explains that the book was written by the writer Aristide Torchia, in 1666, with Lucifer and he was burned at the stake with his entire work. There are only three exemplars of The Nine Gates and in accordance with the legend, its nine engravings form a riddle to conjure the devil. The skeptical Corso accepts the assignment and has to fly to Sintra, Portugal, and Paris, France, to contact the owners Victor Fargas and Baroness Kessler and find the genuine exemplar for Balkan. Meanwhile, he asks Bernie to hide the rare book. Before traveling to Europe, the widow Liana Telfer wants to retrieve the book and has sex with Corso, but ...