Dean Corso (JOHNNY DEPP) is a highly adept, but unscrupulous New York "book detective" who finds rare books for wealthy collectors. Thus, it's quite natural for a scholar of demonology, Boris Balkan (FRANK LANGELLA) to hire him for his expertise. It seems that Balkan has acquired one of three existing copies of "The Nine Gates of the Kingdom of Shadows" from a fellow collector who's just committed suicide.
While the book - a 17th century manual supposedly written by the Devil himself and containing nine engravings that if interpreted correctly can reportedly be used to summon that author -- is now one of Balkan's most prized possessions, he's not completely sure of its authenticity, and thus wants Corso to find the other two copies and prove that they're forgeries.
Although initially reluctant, the large payment Balkan promises Corso eventually convinces him and he sets out to learn about the book as well as track down and compare the two other copies located overseas. It's not long after he begins his investigation, however, that things start to get strange and dangerous. After meeting Liana Telfer (LENA OLIN), the window of the suicidal collector, Corso finds that there's a mysterious blond woman (EMMANUELLE SEIGNER) who seems to be following him, and then discovers that a book friend of his has been murdered along the lines of an engraving from the book.
Once in Europe, Corso continues his investigation that eventually leads him to meet the book dealing Ceniza brothers (both played by JOSÉ LÓPEZ RODERO), as well as Victor Fargas (JACK TAYLOR) and the wheelchair bound Baroness Kessler (BARBARA JEFFORD), who own the second and third copies of the book respectively.
As Corso discovers more revealing facts about the books and finds that he has an ally of sorts in the mysterious blond woman, he must not only contend with Balkan's repeated demands to succeed at his task no matter what the cost, but also with a sinister hit man (TONY AMONI) who's targeted him, as well as surprising revelations about certain characters and their interest in the book.