The Sun has gone out: 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, the Last Redoubt, under siege from unknown forces and Powers outside in the dark. These are held back by a Circle of energy, known as the "air clog," powered from the Earth's internal energy. 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...