I didn't realise when I picked it up that this is the sixth book in a series. Fortunately it can be read alone with no discernible difficulties. I love it. I don't usually read sci-fi, but having enjoyed Left Hand of Darkness I felt I owed it to myself to try more of Le Guin's books.
This one centres around two planets, or rather a planet and a peopled satellite (or moon). Many moons ago (see what I did there), the people of Anarres found it difficult to kill the revolutionary spirit of the Odonians. Instead they sent them to the moon which became an anarchist colony. Some of the residents of the moon, Urras, feel the spirit of individualism has been sacrificed to the needs of the masses. A brilliant physicist, Shevek, is one of them and travels with his theory of The Principle of Simultaneity to a fascist state on Anarres. He clearly sees the differences even though the State try to hide them from him and him from the general populous. The story unfolds along two timelines and on the two planets and shows the problems and benefits of both, something a less talented writer might struggle to describe, but Le Guin handles it all beautifully and the story is immensely satisfying.