Grace Carter, a "source" of magic, has spent the last nine months searching for Maggie Mulvaney, her "catalyst." The joy of reuniting with her partner—and her lover—is thwarted by her worst fear: Maggie remembers neither Grace nor their life together in the Order of Saint Teresa, the centuries-old organization that trained them to be the strongest demon-hunting duo in generations.

When Maggie and Grace unexpectedly come face-to-face with the demon Horde, they are forced to team up once again. As they begin to piece their lives back together, they discover that their memories have been masked by someone within the Order. Should the Horde succeed in their plan, those who have committed their lives to slay worldly demons will be relegated to little more than minions as humans are completely enslaved.

Now, Grace and Maggie must sacrifice everything, possibly even their love, and their lives, in an all-out battle to save humanity.