This novel is a complete rewrite of Clarke's earlier Against the Fall of Night, his first novel, which had been published in Startling Stories magazine in 1948 after being rejected by the editor of Astounding Science-Fiction, according to the author.

In introductions to it Clarke has told the anecdote of a psychiatrist and patient who admitted that they had discussed it one day in therapy, without realizing at the time that one had read one novel and one the other.

He is one of only a very small number of "Uniques", different from everybody else in Diaspar, not only because he has no past lives to remember, but because instead of fearing the outside, he feels compelled to leave.

Alvin continues his quest until he finds out the truth of why the people of Diaspar are so frightened of the external universe and why Lys is so scared of space travel and mechanical things.

It leads them to Shalmirane, the remains of the fortress where the Invaders were fought off with unimaginable weapons, and there they encounter an extraterrestrial creature with a strange robot.

They had originally told Alvin he would be free to choose whether to stay or return, but because the people of Lys had their own insular failure, just like Diaspar, this option was no longer available.

Back in Diaspar, he seeks the help of the Central Computer, which overcomes the Master's block on the robot by creating the illusion of an apocalyptic return of the Great Ones.

Diaspar and Lys are the descendants of those humans who deliberately turned away from the universe in rejection of history's greatest scientific project: the creation of a disembodied intellect.

The Mad Mind had devastated the galaxy and its civilizations before being imprisoned in a "strange artificial star" called the Black Sun.

Vanamonde's ultimate destiny, Hilvar realizes, is to battle the Mad Mind, when it escapes its prison at the end of time.

This departure from the galaxy, leaving it to Vanamonde, was made because they had been contacted by a "very strange and very great" intelligent, extraterrestrial species, which called them urgently to the other side of the universe.