The story follows the adventures of a book dealer, Lucas Corso, who is hired to authenticate a rare manuscript by Alexandre Dumas, père.

Corso's investigation leads him to seek out two copies of a (fictional) rare book known as De Umbrarum Regni Novem Portis ("Of the Nine Doors of the Kingdom of Shadows").

Corso encounters a host of intriguing characters on his journey of investigation, including devil worshippers, obsessed bibliophiles and a hypnotically enticing femme fatale.

While in Madrid attempting to authenticate a previously unknown partial draft of The Three Musketeers, he is summoned to Toledo by Varo Borja, a notoriously eccentric and wealthy collector.

Borja has obtained a copy of a legendary book, Of the Nine Doors of the Kingdom of Shadows, whose author was burned at the stake by the Inquisition.

After several encounters, she attempts to seduce Corso to obtain the draft; when he succumbs to her charms but refuses to surrender the manuscript, she becomes his enemy.

Corso confers with the Ceniza Brothers, book restoration experts with extensive knowledge of forgery.

Corso compares Fargas' copy of The Nine Doors to Borja's, and finds subtle differences in the illustration plates.

Corso visits Baroness Ungern, whose charitable institution possesses the largest occult collection in Europe, including the third copy of The Nine Doors.

They discuss the book's author, before Corso blackmails her with photo evidence of her Nazi sympathies so she will let him examine her copy.

Borja, intending to use the ritual described by the book's true nine plates to summon the Devil and gain ultimate knowledge, has destroyed his entire library to prevent others from following his lead.

The works of Alexandre Dumas, père, the source of the eponymous title, influence nearly every element of the plot.

In 1666, Torchia published De Umbrarum Regni Novem Portis (The Nine Doors to the Kingdom of Shadows), which was in turn based on the Delomelanicon, or Invocation of Darkness, a work supposedly written by Lucifer and that would allow the reader to summon devils.

Several characters' roles diminish, expand, merge, swap or disappear completely, and one of the novel's most important subplots – the Dumas connection – is removed entirely.