This fast-paced mystery thriller revolves around Dean Corso (Johnny Depp), a New York City rare book dealer, makes his living conning people into selling him valuable antique books for a low price, and then selling them for profit.
He meets with wealthy book collector Boris Balkan (Frank Langella), who has recently acquired a copy of The Nine Gates of the Kingdom of Shadows by 17th-century author Aristide Torchia, one of only three copies known to exist. According to legend, the book is Torchia's adaptation of an older book written by the Devil himself and purportedly contains the means to ritualistically summon the Devil and, through him, acquire invincibility and immortality. Balkan believes two of the copies are forgeries.
He hires Corso to authenticate all three copies, and acquire the legitimate copy by any means necessary. Balkan's copy of The Nine Gates had previously belonged to Andrew Telfer (Willy Holt), who committed suicide soon after selling the book to Balkan. Telfer's widow Liana (Lena Olin) wants the book back, as Telfer originally bought the book for her. Liana seduces Corso, but she fails to re-acquire her book. Meanwhile,
Corso's business partner, Bernie Rothstein (James Russo), with whom Corso had hidden the book, is murdered; his corpse is posed in imitation of an engraving in The Nine Gates. Corso travels to Toledo, Spain, where book restorers the Ceniza brothers show him that three of the book's engravings are signed "LCF", which Corso understands to mean that Lucifer himself designed and cut them. Corso goes by train to Sintra, Portugal, to compare Victor Fargas' (Jack Taylor) copy of The Nine Gates to Balkan's.
To his surprise, Corso discovers that the signature "LCF" is found in three different engravings in the Fargas copy, which vary in detail from their counterparts in the Balkan copy. The next morning a mysterious young woman (identified only as "the Girl") (Emmanuelle Seigner) who appears to have been shadowing Corso since Balkan hired him, awakens Corso and leads him to Fargas' house. There he finds the old man murdered and the "LCF"-signed engravings ripped out of his copy of The Nine Gates.
In Paris, Corso visits the Baroness Kessler (Barbara Jefford), the owner of the third copy of the book. The Baroness initially refuses any contact with Corso once she realizes who his employer is, but Corso returns and intrigues her with evidence that the engravings differ among the three copies, and he explains his theory that each copy of The Nine Gates contains three "LCF"-signed engravings, requiring possession of all three copies to have an authentic set of nine for the ritual.
Gaining access to Kessler's copy, he finds the "LCF" on three different engravings, which are also slightly different from their counterparts in the two other copies. Later, Kessler is killed, and the Girl rescues Corso from Liana's bodyguard. Liana steals Balkan's copy from Corso's hotel room. He follows her to a mansion and witnesses her using the book in a Satanist ceremony. Balkan suddenly interrupts the ceremony, kills Liana, takes the engraving pages and his own intact copy, and then flees.
Corso pursues Balkan to a remote castle, which was depicted in one of the engravings, and in a postcard that Corso found in Kessler's copy, and finds Balkan preparing to open the Nine Gates.
After a struggle, Balkan traps Corso in a hole in the floor, immobilizing him and allowing Balkan to perform his summoning ritual with Corso as a 'witness'. After arranging the engravings on a makeshift altar, in a mixed order, Balkan begins reciting a series of phrases related to each of the nine engravings. Balkan then douses the floor and himself with gasoline and sets it alight, believing himself to be immune to the flames. Balkan's invocation appears to fail, however, and he screams in pain as the flames engulf him. Corso frees himself, kills Balkan, takes the engravings and escapes the burning castle.
Outside, the Girl appears to Corso and then has sex with him by the light of the burning castle. She tells him that Balkan failed because the ninth engraving he had used, despite being signed "LCF," was a forgery. Corso, following her directions, returns to the Ceniza brothers' shop. Upon arriving, he finds the store gone and the last piece of furniture being removed, from the top of which falls the authentic ninth engraving with a very different image than the forgery. On it, there is a likeness of the Girl. With the last engraving in hand, Corso returns to the castle it depicts and, having apparently performed all the ritual requirements, crosses the threshold of the Ninth Gate.