The story begins with a ship full of aliens visiting Earth only hours before the Sun will explode, destroying the planet. The mission of the aliens is to try to save as many people and as much of the culture as possible. Normally the galactic civilization does surveys of planets every one million years for new species, but the human race did not exist the last time the survey was done – four hundred thousand years before. However, radio signals had been detected on a planet 200 light years away, indicating intelligent life had arisen.