Corey Fletcher has the ability to travel into the past--so long as he has a metal artifact from a specific point in time, then he can go to that specific point. When, however, he goes into the past and accidentally saves a life, Corey realizes he is something more than a time traveler: he is a "Throwback," someone with the ability to alter life and the whole timeline. He decides to use this ability to go back to September 11, 2001, and stop his grandmother from dying in the terrorist attack of that day, but things go wrong. He ends up in New York City of 1917, lost and alone.
