Andy Reese, an exoplanet astronomer living in the distant future, contacts the Enclave, the last vestige of humanity. To stop the annihilation of the human race by the Seraph, post-biological life forms, Reese is sent back along the timeline to prevent the evolution of the Seraph from computers. The link in time is an Iowa farm girl, Lucy Appel, who has won a national contest to travel to the 1939 New York World's Fair and Professor Herman Geiger, a gifted scientist, writer, and social philosopher. To change history, Reese must avert Geiger's death at the fair. But the Seraph have other plans as they follow Reese back to 1939 to thwart changing history.