Johnny Depp is Dean Corso, "book detective" who wheels and deals old books, the majority of them dealing with magic and the supernatural. He's hired by a rich collector named Boris Balkan (Frank Langella) to acquire the last two texts of an extremely rare manuscript from the Middle Ages. It's rumored that the co-author of this text is Satan himself. He gives Corso the one copy he has--and sends him off to Europe to authenticate the other two copies. Along the way, bad things happen to the owners--and Corso believes that Balkan is responsible. There's also a mysterious young woman (Emmanuelle Seigner) who shows up and she has some seemingly magic abilities. Eventually Corso--and the audience--figures out what is really going on in the last scene, when Balkan acquires what he wants and performs a ritual from the book...
The Ninth Gate is an overall dead serious, creepy movie, along the lines of Roman Polanski's earlier horror movie, ROSEMARY'S BABY. Recommended.