Oh, man, so much. Where to even begin?
Ai has just completed his mission for the Ekumen. Karhide has agreed to parley with the aliens. Tibe has stepped down as prime minister, and in Orgoreyn, Ai's main(ish) man Obsle has risen to power, meaning they'll probably join the Ekumen too. Not too shabby for a day's work.
But Ai still feels lost. His shipmates "all looked strange to [him], men and women, well as [he] knew them" (20.79). He's changed, yo. He's learned to view the world through a more Gethenian point of view at last, but the end result means his no longer belongs to his own world, his own people.
(Side note: This is the problem with the hero's quest. Think about Frodo coming back to the Shire after dropping the stupid One Ring into a volcano—he can take about a year and a half of it before peacing out to the Grey Havens. If you've gone through a life-altering journey, it's really hard to settle back down to your day job.)
So, Ai takes a trip to Estre. There, he presents Estraven's journals to Esvans, Estraven's father. He also meets Estraven's son, Sorve.
Each of the Gethenians has a request from him. Esvans wants to hear the tale of how Ai and Estraven crossed the Ice together. Sorve wants to know how his father died as well as "about the other worlds out among the stars—the other kinds of men, the other lives" (20.103).
In these two requests, we come to a sort of unity, which is a huge theme for the novel*. Esvans wants to hear a Gethen story, but his grandson wants tales about other people and worlds. As Ai noted in the beginning, the stories are not actually so different, they are "all one story" (1.2). So the unity of the Gethen and the other worlds will come from the merging of the stories.
This merging might potentially help Ai find the comfort he could not find in either the Gethenians or the humans after his friend Estraven's death. Although, we can't say for certain, since the story ends with the request for the story. Anyone up for some fanfic?