The Principle of Simultaneity is a scientific breakthrough which will revolutionize interstellar civilization by making possible instantaneous communication. It is the life work of Shevek, a brilliant physicist from the arid anarchist world of Anarres.

But Shevek's work is being stifled by jealous colleagues, so he travels to Anarres's sister-planet Urras, hoping to find more liberty and tolerance there. But he soon finds himself being used as a pawn in a deadly political game.