The themes are very different from most of his robot and 'space opera' stories, and take a clever approach to time paradoxes. Some people consider it his best work, or at least one of the best. (This view is recorded in the Encyclopedia of Science Fiction.)
Despite being written by a highly intelligent former scientist, the novel reflects the state of knowledge of its time, some of which has been superseded. For instance, the power source for the time travellers is referred to as "Nova Sol", a link to the far future being used to tap the energy of the exploding Sun. It is now known that stars of the Sun's size and situation do not explode, and in fact novae are not exploding stars. (Only Type II supernovae are genuine exploding stars.)
As may be seen below, the novel may also be counted as the prequel to the Empire series of novels, which form part of The Foundation Series. He had already included a kind of time-travel in his 1950 novel Pebble in the Sky, though there it was a one-way trip.
The original End of Eternity appeared in 1986 in a collection called The Alternate Asimovs.
The Eternity of the title is an organization and a place which exists outside time. It is staffed by humans (usually male) called Eternals who are recruited from different eras of human history commencing with the twenty-seventh century. The Eternals are capable of traveling “upwhen” and “downwhen” within Eternity and entering the conventional temporal world at almost any point of their choice, apart from a section of the far future which they cannot enter. Collectively they form a corps of Platonic guardians who carry out carefully calculated and planned strategic minimum actions, called Reality Changes, within the temporal world in order to minimise human suffering as integrated over the whole of (future) human history.
A key plot element that emerges quickly as the story unfolds is the relatively static nature of the human societies in the various future centuries, and the repeated failure of space travel in all accessible centuries. We later learn that Twissell (Harlan’s superior) is from "a Century in the 30,000's," yet nothing much is different in that time.
The pivotal moment of the story arrives when the protagonist, Andrew Harlan, an expert in executing Reality Changes, realizes that he is part of a paradoxical scheme to secure the creation of Eternity by sending a young Eternal back in time with the mathematical knowledge to make it possible. Harlan himself is in trouble with the leaders of Eternity. He has been entrapped by one of them into entering into a relationship with a non-Eternal woman, Noÿs Lambent. This was intended merely to prove a point about the effect of Eternity on the individuals from real time who learn of it, but it has the unintended consequence of making Harlan besotted with the woman, so much so that he smuggles her into Eternity, since he has discovered that she will cease to exist in real time when the Eternals make their next Reality Change. Harlan’s whole scheme comes apart when it is revealed the leaders are fully aware of his activities.
Normally the Eternals traverse from century to century within Eternity in a kind of temporal elevator called a kettle. A special version of the kettle has been built, however, for Harlan to dispatch a young Eternal, one Brinsley Sheridan Cooper, back to the 24th century, which lies “beyond the downwhen terminus” accessible via Eternity and its kettle system. Cooper is carefully instructed that he is to teach the principles and technology of time travel to its historic inventor, Vikkor Malansohn, but unbeknownst to Cooper or Harlan, he will actually become Malansohn himself. However Harlan, filled with malice after being confronted with his crimes, scrambles the time settings just as the special kettle departs. Cooper is trapped in the wrong time, so Eternity cannot be created. Unless something is done to change the past, Harlan’s reality, and Eternity, will be erased from existence.
Calming down, Harlan tries to think of a way that Cooper, also adept in the concept of Reality Change, could send him a message to return and retrieve him. Harlan believes that the apparently random target setting he chose on the kettle was the 20th century, and it occurs to him that Cooper was interested in his collection of artifacts from that time, particularly magazines. Perhaps the trapped Cooper would have found a way of leaving his SOS message in one of them.
This is where Asimov’s mistaken “mushroom cloud” appears in the novel. Harlan comes upon an ad for stock tips - All the Talk Of the Market, concealing the acronym A-T-O-M, accompanying a drawing of a mushroom cloud. The year on the masthead of the preserved publication is 1932. Since this predates the first atomic explosion, it must be a coded message from someone from the future. A reality change caused by Cooper.
Before he reveals this discovery to the other Eternals, Harlan exacts a price, his lover is to be returned to him and both will go back to rescue Cooper. Once the couple arrive in 1932, Harlan reveals his last surprise. He has deduced that the woman, Noÿs Lambent, is herself an agent of Reality Change. She is from those centuries the Eternals cannot enter.
She does not deny it. Instead she tells Harlan that her people, who prefer to watch past time rather than travel in it or change it, discovered that Eternity was suppressing the creative individuals in humanity in order to protect the rest. In the end this has the effect of denying humanity's access to the stars, as alien species advance technologically and confine humanity to Earth. Eventually humanity will die out, millions of years in the future, leaving an empty Earth. However, if Eternity could be prevented from being created, humans would leave Earth and colonize the stars. Thus they cut themselves off from Eternity and began to plot its demise.
Noÿs Lambent reveals that in order to make Eternity improbable, Harlan needs only to decide to leave Cooper stranded in 1932. She also intends to send a carefully-worded letter to Italy, causing a man (presumably Enrico Fermi) to "begin experimenting with the neutronic bombardment of uranium". This will start a chain of events which will lead to the first atom bomb in 1945. In the reality known up to that point, atomic power was not discovered until after Eternity was created. Acquiring the technology sooner to supply energy, humanity will gain an additional time advantage to leave Earth, and maximize the probability of building a Galactic Empire.
Harlan at first intends to kill Noÿs and carry out his mission, but in comparing her story to that of the freakish and occasionally inhuman Eternals he has encountered, Harlan confirms his lingering suspicions that Eternity has been wrong for humanity. At the very moment he decides to help her, a Reality Change occurs and the 'kettle' linking them with Eternity vanishes into thin air.
The encoded message concerning atomic power is not entirely convincing within the bounds of Asimov’s concept - H. G. Wells could conceive of both atomic power and atomic weapons in The World Set Free (1914), and he was not alone in this. Fermi's work was important, but the lack of it does not explain why no atomic bomb was used - even as demonstration, up to the 30th century (the description of the 24th century includes nuclear energy, which means that the delay in the development, while crucial, was not above 4-5 centuries, or perhaps even affected the mindset of the society instead of the technological progress). This final plot twist does nevertheless signal to the reader what was previously hidden, in “whodunit” fashion - that the new future being created by Harlan and Noÿs corresponds to the reader’s future, and that 'Eternity' was actually a dead end.
The novel’s closing provides the reader with a reassuring perspective for what future will come into existence:
The woman from the far future does explain that her people are working to ensure that a Galactic Empire becomes a certainty. The hint could mean that a real record got through but was garbled, confusing the Eternals with their unnamed enemies. Noÿs says "we will remain to have children and grandchildren, and mankind will remain to reach the stars". If they had passed on some knowledge, they might have been selective and not mentioned the dangerous alternatives.
The original unpublished End of Eternity is clearly a different future from that of the Foundation. But Asimov says in his story-postscript that he had some idea of a bridge in the published version.
Asimov placed a hint in Foundation's Edge many years later, that the Eternals might have been responsible for the all-human galaxy (and the development of humanity on Earth) of the Foundation Series But that interpretation is disputed. Asimov himself mentions the disparity. The human-like robots may have been intended to play a part. It is one of the loose ends that he may have planned to clean up, but which his death obviously prevented.
The novella Great Work of Time by John Crowley shares the basic outline of "The End of Eternity" - i.e. a secret society of well-meaning time travelers bent on remodeling history, and a young man recruited into the society in order to make a specific change that would bring this society itself into being. However, the details are completely different from those in Asimov's book: on the one hand, Crowley's time travelers are far more "localised" than Asimov's, being mainly concerned to "redesign" the Twentieth Century, prevent the World Wars and perpetuate the British Empire; on the other hand, the unforeseen results of their interference are catastrophic in a far more fundamental way than in the Asimov book.