When Kate Pierce-Keller’s grandmother gives her a strange blue medallion and speaks of time travel, sixteen-year-old Kate assumes the old woman is delusional. But it all becomes horrifyingly real when a murder in the past destroys the foundation of Kate’s present-day life. Suddenly, that medallion is the only thing protecting Kate from blinking out of existence. Kate learns that the 1893 killing is part of something much more sinister, and Kate’s genetic ability to time-travel makes her the only one who can stop him. Risking everything, she travels to the Chicago World’s Fair to try to prevent the killing and the chain of events that follows. Changing the timeline comes with a personal cost, however—if Kate succeeds, the boy she loves will have no memory of her existence. And regardless of her motives, does she have the right to manipulate the fate of the entire world?