Nestled between San Francisco and the heart of California wine country is the quiet community of Del Sombra. Among its many residents are Eliot and Fiona Post, twin fifteen-year-olds who want nothing more than to get out from under the oppressive force of their grandmother, Audrey. Audrey controls every aspect of Eliot and Fiona's lives: she homeschools them, forces them to work at a local pizzeria, even attempts to control their interests and actions through an ever-growing list of rules. On their fifteenth birthday, they learn why. They are the offspring of the eldest Immortal Fate, Atropos, and the Infernal Lucifer, and their very existence is a threat to an ancient neutrality treaty between gods and fallen angels.
When news of their existence hits each family, a cold war erupts, each side vying for control of this new breed. In order to establish their proper place and rightful allegiance, the Immortals and Infernals devise tests for Eliot and Fiona. The gods devise three heroic trials inspired by urban legends, taking them into deeper and more dangerous pockets of mythology incarnate in the modern world. The Infernals fashion three diabolical temptations for the children, each one an attempt to forever isolate brother from sister.
Eliot and Fiona will need to band together if they are to have any hope of staying alive, for allegiances are never quite what they seem in this ancient battle for supremacy. And they just may need to call for their parents' help if they hope to stop these secret worlds from exploding into an end-all battle...