For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Mission Improbable
Act 1 establishes the characters, the setting, and the gist of the story, and for this novel, that's Chapters 1 to 5. Here, we meet Genly Ai and Estraven and come to understand Ai's mission as Envoy for the Ekumen. We come to see Estraven as a bit of a mystery, one we're going to solve later in the story. As Ai travels through Gethenian society, we learn about the Gethenians as well as their culture and their planet.
Act I lasts until the point of no return. For Ai, the point of no return is when he decides he needs to head to Orgoreyn to complete his mission. Of course, he doesn't realize exactly what he's walking into.
Note: The "point of no return" will be different in a Three-Act plot structure than a classic plot structure. The story changes ever so slightly depending on how you look at it—which is pretty cool if you ask us.
Act II
Orgoreyn Bound and Down
Welcome to Chapters 6 through 14. In a traditional three-act structure, the second act is reserved for more intense action by putting the main character in the worst possible situation. Their darkest hour, if you will. And, dude, does Ai have a dark hour or what.
The Orgoreyn government wines and dines Ai. Meanwhile, a separate government department, Sarf, has other plans. They have him sent to a forced labor camp where he undergoes extreme emotional and physical stress/torture. Also during this Act, we learn more about Estraven and his plans for Ai, thickening the plot soup.
Act II ends with Estraven breaking Ai out of the camp, and the two preparing to escape Orgoreyn by traveling 840 miles of inhospitable wilderness.
Act III
A Snowball's Chance in…a Really Hot Place
Now we're at Chapters 15 to 20. Third Acts consist of falling action leading to the end of the story, or resolution. In the case of The Left Hand of Darkness, that's Ai and Estraven's journey across the Ice and the wrap-up.
As Ai and Estraven travel the Ice, the two become friends. The mystery of Estraven is mostly solved as Ai comes understand Gethenian society better. There's still plenty of action, but the story's conflicts are wrapping up nicely while our questions are being answered. When Estraven dies and Ai convinces the King to join the Ekumen, the final pieces slot into place. We can put the book down and enjoy that lovely feeling of having completed a wonderful book.