The Left Hand of Darkness is set in the "Hainish" universe, which Le Guin introduced three years earlier in Rocannon's World, her first novel. The series describes the interplanetary expansion started by the first race of humanity on the planet Hain, leading to the formation of the League of All Worlds, and eventually expanding to the eighty-three world collective called the Ekumen.
This novel takes place many centuries in the future – no date is given, though the year 4870 has been suggested. An envoy, Genly Ai, is on a planet called Winter ("Gethen" in the language of its own people) to convince the citizens to join the Ekumen. Winter is, as its name indicates, a planet that is always cold, and its citizens are "ambisexual," spending the majority of time as asexual "potentials." They only adopt sexual attributes once a month, during a period of sexual receptiveness and high fertility, called kemmer, in which individuals can assume male or female attributes, depending on context and relationships. These conditions have affected the development of civilizations on Winter. Among other things, the planet has never known war.