A 17th-century gentleman, mourning the death of his beloved, Lady Mirdath, is given a vision of a far-distant future where their souls will be re-united, and sees the world of that time through the eyes of a future incarnation. The Sun has gone out and the Earth is lit only by the glow of residual vulcanism. The last few millions of the human race are gathered together in a gigantic metal pyramid, nearly eight miles high - the Last Redoubt, under siege from unknown forces and Powers outside in the dark. These are held back by a shield known as the "air clog", powered from a subterranean energy source called the "Earth Current". For millennia, vast living shapes-the Watchers-have waited in the darkness near the pyramid. It is thought they are waiting for the inevitable time when the Circle's power finally weakens and dies. Other living things have been seen in the darkness beyond, some of unknown origins, and others that may once have been human. The narrator sets off alone into the darkness to find the girl he has made contact with, hoping that she is the reincarnation of his past love.