It is staffed by humans called "Eternals" capable of traveling “upwhen” and “downwhen” within Eternity and entering the conventional temporal world at almost any point of their choice. 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.
Andrew Harlan, an expert in executing Reality Changes, falls in love with a non-Eternal as he comes to understand his part in 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...