A rare book dealer, specializing in finding valuable volumes for wealthy clients, is hired by a renowned book collector to authenticate a 17th-century manual purported to summon the devil. This manual, known as "The Nine Gates of the Kingdom of Shadows," is said to hold the secret to gaining immense power and is linked to the dark arts. The book dealer embarks on a journey across Europe to compare the collector's copy with the only two other copies known to exist. His mission is not only to verify the authenticity of the book but also to uncover any differences between the volumes that might reveal the true power of the manual.

As he travels from Spain to France, and then to Portugal, the book dealer encounters a mysterious woman who seems to follow him on his quest. He becomes entangled in a web of murder, intrigue, and occult rituals, which reveals that the stakes of his mission are higher than he initially realized. Along the way, he is confronted with puzzles and codes hidden within the pages of the manual, each discovery drawing him closer to understanding the book's true essence. Unbeknownst to him, his journey attracts the attention of a shadowy group with their own vested interest in the manual's secrets, setting the stage for a confrontation between ancient forces.