During an experiment into time travel, a scientist finds himself trapped in the past, "leaping" into the lives of different people, sorting out their problems and changing history in hopes of getting back to his own life in the present.
Sam's leap takes a dramatic turn when he begins leaping randomly throughout the life of Lee Harvey Oswald, accused assassin of President John F. Kennedy. Matters are complicated further when Sam's mind becomes tangled with Oswald's.