Twenty-five years after Yellowstone erupts, a handful of humans struggle to survive in the ruins of their shattered civilization. Among them is Ciarán Ó Corraidhín, an aging American mercenary trapped in the Caucasus. When he meets Ani Ohanyan, a beautiful young woman smuggling Mamikonium, the lone drug that allows humanity to breathe the ash-filled air, Ciarán finds himself questioning all that he knows and believes.