Harlan's job is that of Technician: one of the secret field agents responsible for actually effecting the desired Changes calculated by the other Eternals, such as the Computers (a rank, not a machine). Computer Laban Twissell is Harlan's direct boss. He has, for some mysterious reason, taken Harlan under his wing. But he is happy to lend Harlan out for a quick assignment in the 482nd century.

But when Harlan dips physically into this era of Reality, he has the senses-shattering experience of falling in love with a local, a seductive and compliant woman named Noÿs Lambent. Besotted by this woman—his first lover—Harlan is shattered to learn that an upcoming Reality Change will wipe her from existence. He convinces her to step into Eternity with him, then hides her in a segment of the system up in the empty Hidden Centuries.

Harlan returns to work, a nervous wreck. To make matters worse, bits and pieces of a master conspiracy that had long been troubling him start to fall into place. He learns that Computer Twissell and Harlan's own protégé, Brinsley Sheridan Cooper, are not what they seem. In fact, Cooper, Twissell and Harlan form the core of a time paradox. The three of them must perform predestined actions for Eternity to continue existing. But what if Harlan chooses to default? Will Eternity cease to exist? A huge responsibility for any man.

And then Noÿs proves to be a posthuman from the Hidden Centuries, manipulating everyone!

Master of the masters of space and time

As John Clute remarks in The Encyclopedia of Science Fiction, "the 1950s ... was the heyday of the 'time police' story, in which vast manifolds of Alternate Worlds were routinely patrolled by cunning secret agents or historical conservationists." Works by Anderson and Leiber spring immediately to mind, and Asimov's certainly belongs squarely in that camp. So far, so unexceptional. But Asimov's book—clever and intricate and expertly plotted as the others—has a few unique and surprising qualities that distinguish it from its peers.

First off, just like Sloan Wilson's famous The Man in the Gray Flannel Suit, which coincidentally appeared the very same year, Asimov's book is a reflection and critique of the Organizational Man. Eternity is portrayed as something larger than the individuals that compose it, an entity that knows better than anyone what's good for humanity, one that can't be contravened. There's almost an air of early Vonnegut—think Player Piano (1952)—to this aspect of the book.

Second, Asimov employs definite and unexpected noir tropes: the virginal, upright stiff seduced by the femme fatale and turned against society; corrupt institutions; flight and hiding; betrayals. His first SF-mystery hybrid, The Caves of Steel, had appeared the previous year. But instead of a "police procedural," Asimov could be seen experimenting with John D. MacDonald/Gold Medal PBO style and themes. This book's surprising affinity with Bester's The Demolished Man (1953) (albeit not stylistically) is also worthy of note.

Finally, Asimov's flirtation with the tropes employed by A.E. van Vogt and Charles Harness—reality as a function of perception, a kind of Heisenbergian anthropic principle—is startling for an author deemed ultra-rational and scientific.

The effects of this influential, seminal book echo to the present, in the works of such writers as Greg Egan, John Varley, Kage Baker and Greg Bear. And some fans even maintain that it forms the necessary prelude to Asimov's own Foundation series, explaining why humans alone populate the galaxy, although it was never officially tied into that mythos.