William O'Donnal thought he'd given up his soul when he traded his place in Heaven for two more minutes with his dying wife. Instead, he finds himself in the service of the Arch-Seraphim Gabrielle, tracking down daemons, demons, and other hellspawn. When one of the other three Arch-Seraphim goes missing, however, the search for answers takes him and his angelic partner across the world.

A follow-up to Robert M. UnKauf's debut novel Ascension, Guardian continues to follow William O'Donnal as he finds himself embroiled in what is beginning to appear more and more like a revolt within the Seraphic ranks. Banned from dealing with Gabrielle and Michaela by an order from the Circle, Will has completed his training with Uriel. Unexpectedly, however, he finds himself challenged by words not heard in two thousand years: "Guardian. Attend."