This story rated three stars most of the way through, but the great ending catapulted it up to four. Asimov creates a very complex future world where 'eternity' has been created, a sort of laboratory outside of time where technicians may travel through time and alter history to guarantee a better future for humanity. Their eternity stretches from the 27th century to the 70,000th.

So the ideas are fascinating, yes, but the main character is kind of a hot-headed, simple-minded jerk. It's tiresome spending the whole book with him. His falling in love and deciding to destroy 'eternity' to save this woman is handled briefly and poorly. And yet--it turns out this book is something of a mystery, too. Things are not what they seem. And the ending turns out to be really wonderful, encapsulating the hope and wonder and everything else that science and science fiction meant to people in the '50s.