Genly Ai, the envoy of Ekumen, a federation of worlds, is living on Karhide, also known as Winter: "the dominant factor in Gethenian life is not sex or any other human thing: it is their environment, their cold world. Here man has a crueler enemy even than himself."
Genly seeks an alliance with the nations of Gethen. To the Gethenians, he is an alien, although he doesn't look too different from them. But they are quite unlike in one particular way: "Cultural shock was nothing much compared to the biological shock I suffered as a human male among human beings who were, five-sixths of the time, hermaphrodite neuters." In other words, "the mother of several children may be the father of several more".
For his belief in Genly's mission, the Prime Minister of Karhide, Estraven, is exiled. He enters neighbouring Orgoreyn to seek shelter, Genly's path leads there too, and it is unfortunately, the wrong one for Genly. The Orgota, playing a political game of sorts, imprison him, refusing to believe that he is an emissary from another world. Estraven rescues him from the work farm and they trek for months across the frozen landscape to Karhide.
The Left Hand of Darkness is a tale of an unexpected friendship. Yes it's about politics and gender issues but it's too cloudy and sleepy a day to start on all that. You can google 'left hand of darkness' and find out a whole lot more, I'm sure, if you're so inclined. I definitely enjoyed the book, although I have to also add that Le Guin's Earthsea series is among my all-time favourite books, so perhaps I went into this book expecting to like it. And I did. It was written from an anthropological view, an observation of the world so different from ours. I especially liked Ursula K Le Guin's introduction to the book, where she writes:
"In reading a novel, any novel, we have to know perfectly well that the whole thing is nonsense, and then, while reading, believe every word of it. Finally, when we're done with it, we may find - if it's a good novel - that we're a bit different from what we were before we read it, that we have been changed a little, as if by having met a new face, crossed a street we never crossed before. But it's very hard to say just what we learned, how we were changed."
PS: On Le Guin's website, she posts a rejection letter for her manuscript of The Left Hand of Darkness.