The novel incorporates a novelette of the same name, first published in 1953.
Historia Galactica, Vol. MXLIII
Centuries ago, after the fall of Western civilization, when Earth's resources were consumed, entire cities cut loose from the home planet to roam the Universe.
Earth, the sleepy capital of the Universe, was deserted - a garden planet. It was known only as a legend in many corners of space - a green myth floating untold millions of light years away.
Then, as foretold by cyclic theories of sociahistorians, colonies of flying cities reverted to the laws of the jungle - to barbarity, cruelty - naked force! These outlaw cities banded together to march on Earth - and obliterate the birthplace and last stronghold of universal culture!
The only hope for the universe lay in two men - one young and vigorous, the other over one thousand years old!