The End of Eternity

Andrew Harlan is an Eternal, a man whose job it is to range through past and present Centuries, monitoring and, where necessary, altering Time's myriad cause-and-effect relationships. But when Harlan meets and falls for a non-Eternal woman, he seeks to use the awesome powers and techniques of the Eternals to twist time for his own purposes, so that he and his love can survive together.