After decades of stability, climate stresses, never far from the surface, are bringing droughts, crop failures, and massive storms. The world's end--avoided once, centuries before--seems likely to succeed the second time. Scientist Sarah Nahanni has a possible solution, but the math is daunting and the number of mathematicians willing or even able to solve the equations is very small. With the ancient satellites failing and the roads filled with hostile armies, the path to recovery seems lost. Far to the south, Ivan Rodriguez, an unlikely genius, is on the run from the death squads of his feudal overlords, his head full of fears for his family and mathematical dreams of a better future. He holds the key to Sarah Nahanni's problem and would help--if only he knew she existed and he could reach her in time.