The Eternity of the title is an organisation which exists outside time. It is staffed by humans (specifically male humans only) called Eternals who are recruited from different eras of human history starting in the twenty-third century. The Eternals have the capability of ranging up and down time and entering the conventional world at any point of their choice. Collectively they form a corps of Platonic guardians who carry out carefully planned actions within the temporal world in order to minimise human suffering as integrated over the whole of human history.

This has the effect of denying humanity's access to the stars, as alien species advance technologically and confine humanity to Earth. The story's protagonist, realizing this, sabotages the initial creation of the Eternals, allowing humanity to explore the stars.