On the planet Abeth, a narrow Corridor of green land is surrounded on all sides by ice plains where only the strong survive. Ice triber Yaz has completed a perilous journey and arrived at the Corridor, and it exceeds and overwhelms all of her expectations. Everything seems different but some constants remain: her old enemies are still two steps ahead, bent on her destruction. She makes her way to the Convent of Sweet Mercy, where nuns train young girls who show the old gifts, but like the Corridor itself the convent is packed with peril and opportunity. Yaz has much to learn from the nuns--if they don't decide to execute her.

The fate of everyone squeezed between the Corridor's vast walls, and ultimately the fate of those laboring to survive out on ice itself, hangs from the moon, and the battle to save the moon centers on the Ark of the Missing, buried beneath the emperor's palace. Everyone wants Yaz to be the key that will open the Ark - the one the wise have sought for generations. But sometimes wanting isn't enough.