Synopsis: If the universe is infinite, it follows that there may be somewhere real physical worlds that duplicate those of the imagination. And when Tom Carson caught sight of the third planet of 82 Eridani he recognized at once its resemblance to that imaginary Mars called "Barsoom" of the ancient novelist Burroughs. Of course there were differences, but even so this planet was ruddy, crisscrossed with canals, and its inhabitants were redskinned, fought with swords, and had many things superficially in common with the fantasy Mars of the John Carter adventures. But there were indeed vital variations that would eventually trip up the self-deceived science-fiction-reading travellers from 24th Century Earth. (adapted from the back cover of the first edition)