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.

One of Isaac Asimov's SF masterpieces, this stand-alone novel is a monument of the flowering of SF in the twentieth century. It is widely regarded as Asimov's single best SF novel.