This world, the planet Gethen (Winter) is also a world which has no war: its two main nations - a feudal kingdom and a state capitalist country with a secret police and prison camps - keep an uneasy balance while constantly conspiring against each other. The notion of balance between opposites is at the centre of Le Guin's novel. Gethen's best-known poem starts with the lines "Light is the left hand of darkness / And darkness the right hand of light," and the novel is built on the conflicts, truces and precarious balances between binary opposites: loyalty and betrayal, fear and courage, strength and weakness, cold and warmth.
Gethen is seen mainly through the eyes of Genly Ai, an Earth-born, male representative of the galaxy federation Ekumen, sent to Gethen to persuade the two superpowers to agree to the planet becoming a member of Ekumen. Caught in a web of conspiracies, he ends up in a prison camp, and is rescued by his only friend and ally, the disgraced former Prime Minister Estraven. Together, Genly Ai and Estraven escape from both superpowers across 700 miles of ice; their three-month trek allows them to experience in depth, and finally come to love, each other's "otherness".
Le Guin is a gifted writer, and manages to move her readers with her metaphors and flowing sentences rather than overtly didactic speeches. The novel is genuinely moving - more in the second part, the long journey of escape and learning, than in the first - but it has two serious problems. Although Gethenians are androgynous, Le Guin shows them mostly as male, in the public rather than in the private sphere, and uses masculine forms to refer to them. She also, after making Gethenians androgynes, does not allow them any sexual activity or attachment which is not heterosexual and connected to reproduction. It is to her credit that, re-examining her novel in 1976 and again in 1989 (in an essay with the revealing title "Is Gender Necessary?"), she acknowledged these limitations and stated that, given the chance to do it all over again, she would have used neutral pronouns or alternated masculine and feminine forms, and she would have given her characters the option of homosexuality.
Postscript: Ursula Le Guin includes homosexual Gethenian unions and other issues such as female infanticide and atomic apocalypse in subsequent stories. These stories have been published in a new collection, "The Birthday of the World and other stories", Gollancz, 2002.