On the planet Abeth, a narrow Corridor of green land is surrounded 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 Sweet Mercy Convent, where nuns train young girls who show the old gifts, but like the Corridor itself, the convent is packed with peril and opportunity. The fate of all those squeezed between the Corridor's vast walls, and ultimately the fate of those labouring to survive out on the ice, hangs on the moon, and the battle to save the moon centres on the ark of the Missing, buried beneath the emperor's palace. Everyone wants Yaz to be the key that will open the ark. But sometimes wanting isn't enough...