Twice a century, the world of Duranaya grows new surfaces from the top of its mountain, casting old ones into darkness and forcing a constant upward migration of the nation. But there are those who live in the pitch-black Undersurface ... and they have come to claim the sunlit world, with unlikely power and allies.

In the midst of the invasion, Whissen, a monk afflicted with an assassinated mind, crosses paths with Reph, a huntress with a criminal past. They have little in common, but they do share a predator. One who has rediscovered their scents and knows their hidden, guilty weaknesses.

The invasion pauses for no one. Reph's aging father is trapped in a hopeless siege, and time is short. Together, Whissen and Reph must cross enemy territory before Reph's father is lost, chased by a killer who plays with her food. Can they turn their weaknesses into strengths before their pursuer does it for them?