In 1581, Dr. John Dee received communications from angelic beings, documenting a divine language called the Claves Angelicae. Hidden within his encrypted papers lay evidence of a forbidden forty-ninth key. When Nathan Cross buys a Phoenix storage unit and discovers these stolen artifacts, he becomes the target of two ruthless forces: the Hermetic Order of the Golden Dawn, who want their materials back, and the Astrum Obscura, who will do anything to open the forty-ninth gate. After his best friend is murdered, Nathan flees with Dr. Olivia Ashmore, an Oxford scholar who understands the artifacts' true power. But as the artifacts bond with Nathan. granting supernatural knowledge while corrupting his soul, she realizes the gate doesn't lead to heaven.

It leads to somewhere else. Hunted across three continents, their journey ends beneath Antarctica's ice, where an ancient gateway waits. Olivia must choose between saving the man she's come to love or saving the world from unmaking itself.