The captivating novel that is considered by many to be the most important work of Isaac Asimov, the author who shocked with his groundbreaking ideas and forever changed the world of science fiction.

Andrew Harlan is an Eternal, a man who has abandoned his home time to help improve humanity. He enters the various Ages of the past and the distant future, identifying human "errors" and, where necessary, making Reality Changes - always calculated with absolute precision and skill. The simple movement of an object a centimeter closer can completely abolish a war or eliminate an epidemic from the history of Time. However, what is the ethical cost of such a change?

Harlan will face the ultimate dilemma and will be called upon to risk everything when he falls in love with Noys Lambent, a Timer, and together they create a chrono-paradox.