The novel opens with a parade: colors, marching, instruments, the Macy's Day works. But Genly Ai isn't on the planet Gethen to enjoy parades. He's there to convince the Gethenians to join the Ekumen—think the United Nations, but spanning the universe.
And things could be going smoother. For starters, the Gethenians are androgynous beings, neither male nor female. This aspect makes their entire culture distinct from any Ai has ever encountered, meaning navigating their politics and social mores is tricky-tricky business.
And then there's Estraven, prime minister of the country Karhide (on planet Gethen) and the guy (guy?) vouching for Ai to the country's King Argaven. Too bad Ai just can't figure this dude out. Like, why is the Gethenian so cold and remote? What kind of power does Estraven hold over Argaven? No clue.
Ai receives a summons from King Argaven the same day Estraven is run out of town for being a traitor. Not good. Luckily, King Argaven doesn't blame Ai for Estraven's mistakes—but that still doesn't mean he's going to join the Ekumen. Because he's not. It's back to square one for Ai.
Time for a vacay. Ai heads to the Fastnesses of Karhide and meets Faxe the Foreteller. He asks Faxe for to prophesy whether or not the planet Gethen will join the Ekumen in five years time. Faxe checks his Magic 8-Ball and answers yes, so Ai decides to restart his mission. With Karhide a total bust, it's off to a different country, Orgoreyn, to try his luck.
Now things start getting interesting, because war is brewing between Karhide and Orgoreyn. Oh, and Estraven is heading for Orgoreyn too.
The Orgoreyn Commensals, the 33 people who run the government, openly welcome Ai into their country. Things seem to be going great for Ai on the surface. He's set up in a comfy apartment and has friends in high places (the government). The Commensals even seem to be working with him in ways King Argaven never did.
Luckily Estraven is paying attention, because he knows that Ai has some way powerful opponents in the Orgoreyn government. Estraven tries to warn Ai, but Ai's had just about enough of Estraven's secrecy and ignores him.
Bad move. In the night, Ai is whisked away by the police and sent to the Volunteer Farm, which comes across as more "Enslave & Torture Farm." There, Ai is at the brink of insanity and death when is Estraven swoops in awesome-style and saves him. Together, the two make a daring escape over some of the most inhospitable landscape this side of Antarctica and, in the process, become friends who finally understand one another.
They barely manage to survive the trek to the Orgoreyn/Karhide border. There, Ai signals for his ship to come down, proving his story once and for all. At the same time, Estraven is betrayed by a friend and killed while trying to escape back into Orgoreyn.
But it's all well that ends well (unless you're Estraven). Ai accomplishes his goal, King Argaven accepts the Ekumen's proposal, and Ai knows the truth of his friend's sacrifice.