There is a group of people (only males) who are called The Eternals. They live outside of ordinary time and space in a man-made construct called Eternity. The Eternals can move back and forth between Eternity and Earth, entering into any time period of Earth's history. Their mission is to make Reality Changes, changes in the course of human history that will result in an improved Reality. They try to do this with the help of computers that can predict how even subtle changes will alter Reality. There is an art to finding the minimal intervention that will result in a desired Reality Change. There is a special change called "The Minimum Necessary Change".
The main character, Andrew Harlan, has a natural ability to find "The Minimum Necessary Change". Harlan becomes the special Technician of Senior Computer Twissell, the head of the All When Council and leader of Eternity. Among the various professions available to Eternals there is a hierarchy of respectability. Technicians make the Reality Changes, the "dirty work" of Eternity. Because of his work, Harlan is shunned by other Eternals and sometimes called an "executioner", or worse.
Harlan's hobby is study of Earth's history in the centuries before Eternity was created, the so called Primitive era. Twissell orders Harlan to teach Primitive history to Brinsley Sheridan Cooper, an Eternal who was was admitted to Eternity under unprecedented circumstances.
Harlan is ordered to make a series of Observations on a particular period of Earth's history that is scheduled to undergo a Reality Change. While working on this project, Harlan falls in love with a woman called Noys Lambent. Harlan becomes a criminal by removing Noys from Time and protecting her from the scheduled Reality Change by hiding her within Eternity. Harlan's crime comes at a crucial point in the history of Eternity itself.
Harlan learns that Cooper must be sent back into the Primitive with information from the future in order to make possible the creation of Eternity. Harlan, fearing that his crime has been detected and believing that Noys was taken away from him, decides to take revenge and destroy Eternity. Harlan manages to disrupt Cooper's travel into the Primitive, stranding Cooper in the 20th century.
Twissell and Harlan come to an agreement that will allow Eternity to be saved. Harlan will be allowed to take Noys back in time to the 20th century. Harlan must rescue Cooper and help re-direct him to the correct point in time from which he can assure the invention of Eternity. Harlan will then be allowed to live happily ever after with Noys. Harlan and Noys reach the 20th century, but Harlan tries to kill Noys. Harlan has realized that Noys is a time traveler from the far future who is trying to destroy Eternity. However, before Harlan can pull the trigger and blast her, Noys explains that because of the existence of Eternity, humans never develop space travel and the species simply dies off in the far future. Noys explains that they must prevent the creation of Eternity and also help start nuclear physics in the 20th century, making possible atomic technology that will give a crucial boost to the development of space travel. Harlan realizes that Noys is correct and so they destroy the Eternity, honoring the title of the novel.
This novel is the second version of a previous novel with the same name. The original writing was shorter. Asimov came up with the idea for this story by reading an old Time magazine. He thought he had seen a toadstool, very similar to the atomic bomb. The problem was that the magazine was fifteen years before Hiroshima, according to Asimov's recalling of the anecdote. He thought of a way for a time traveller to send a message to the future in case he were trapped in the past. Then, Asimov started to write and came with a 25.000 words story in 1954. It was a great success as its longer version.