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. Corso's travels take him to Madrid (Spain), Sintra (Portugal), Paris (France) and Toledo (Spain).