By the twenty-first century the world was drowning in its own population.

A solution had to be found quickly - and it was, in gravi-power. Wonderful, unending source of power - only the scientists knew that its use was reducing the Earth''s distance from the sun at a dangerous rate. But if another planet''s gravi-power could be harnessed . . .

An expedition was launched to Mars, known to be uninhabited - except that a woman was wandering around its surface who claimed she had come from another galaxy to warn Earth of a terrible menace, and there was a huge poly-hedron of metal emitting a force very much like that of gravipower.

Someone else had discovered it! A thousand years ago, or now? Friend, or enemy?

(First published 1965)