Long before the world of the Ganymeans blew apart, millennia ago, the strange race of giants had vanished. No one could discover their fate, nor where they had gone, nor why. There was only a wrecked ship, abandoned oon a frozen satelite of Jupiter. And now Earth's scientists were there, determined to ferret out the secret of the lost race. Then suddenly, spinning out of the vastness of space, a ship of the strange, humanoid giants returned. They brought with them answers that would alter all Mankind's knowledge of human origins and startling revelations from the past that would have biologic reverberations to the end of time...