Here is the gist: In the future a collective of humanoid worlds has formed a cultural alliance knows as the Ekumen* that seeks out and invites other humanoid cultures into the alliance. The invitees are thoroughly studied before the Ekumen sends a lone cultural emissary to make the invitation. On the icy planet of Winter, where every human is simultaneously both genders while being neither, warfare is non-existent despite the fact that there are individual countries and complex hierarchies of respect. In the midst of a tense political climate the lone emissary seeks to understand Winter's culture(s), taboos, and intrigues while working towards convincing the entire planet to accept a role in the vast alliance that exists outside of their world.
Even as I type this I realize just how fantastical this all sounds and it doesn't at all do justice to the philosophical questions that underpin the narrative. What would happen if every woman could also be a man? How would equalizing the role of pregnancy effect the potential for aggression in a society? If no one was ever sexually frustrated how would that effect cultural progress? Is religion a universal need? Is truth?
One of the questions that you sit with for the entire book is the rational behind a huge, highly sophisticated alliance, choosing to send only one emissary to a planet in order to impart the knowledge that the inhabitants of that planet are not alone in the universe. There is a lovely quote near the end of the book that unpacks this idea a bit, which is shared by the emissary with his friend:
"I thought it was for your sake that I came alone, so obviously alone, so vulnerable, that I could in myself post no threat, change no balance: not an invasion, but a mere messenger-boy. But there's more to it than that. Alone, I cannot change your world. But I can be changed by it. Alone, I must listen, as well as speak. Alone, the relationship I finally make, if I make one, is not impersonal and not only political: it is individual, it is personal, it is both more and less than political. Not We and They; not I and It; but I and Thou. Not political, not pragmatic, but mystical."
A similar unfolding of the mystical also occurs in Le Guin's marvelous A Wizard of Earthsea, where the true name of things holds incredible power, and the author also uses the complexity of religious belief on our world as inspiration for the dreams and demons of the fictions she constructs.
In my mind, a good work of sci-fi is more about reality than fiction. It is a thought experiment not at all dissimilar from those the ancient Greek teachers used to open the minds of their pupils during the Golden Age of western philosophy. In the formulation of a question you are left to consider how that one question is really just a invitation to so many more.
*a word obviously derived from ecumenical, which I've always associated with the body of Christian churches, but more precisely can be defines as belonging to the whole, universal- a word that in itself comes from the Greek word for belonging to the inhabited Earth