Superficially, this novel is the story of Genly Ai, first ambassador ("Mobile") of Hain, to the people of the kingdom of Karhide on the planet they call Winter, and what happens to him there when he becomes entangled in an accusation of treason. He is imprisoned and escapes, and there is a considerable amount of suspense and action as Genly navigates his way around Winter and its kingdoms, eventually finding his way back to safety.
I say "superficially" because this book really has nothing to do with any of those things at all. They are merely the framework that Le Guin uses to arrange the true story upon. .
The Hainish people, Ai's people, are human as we would recognize them. Le Guin's future universe is populated by the Hainish and non-Hainish, and her best work is typically a story of what happens when Hainish ambassadors go into the field -- become "Mobiles" -- as first contacts to non-Hainish worlds. The Left Hand of Darkness is no exception. The true story is how the experience of immersion in a wholly different culture transforms the Mobile.
Le Guin's worlds are intricately drawn and usually radically different from our own. Gethen, or, as the natives call it, Winter, is an aptly named frigid planet which endures 10-foot snows for the majority of its year. It has a population unlike any encountered in any other science fiction work. It is a genderless world, or, more precisely, its inhabitants are androgynous most of the time. Periodically they enter a phase called kemmer, and in this phase they can assume either male or female characteristics. Therefore it is possible for a Gethenian to both bear young and to impregnate a partner. But the Gethenians do not even think about these relationships outside of kemmer, and gender references are completely absent in their society.
This absence of gender is simply maddening to Ai, who cannot accustom himself to the duality of the Gethenians.