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
The three acts of Brave New World can be marked roughly by the changes in setting. The first act, therefore, runs from Chapter 1 up though Chapter 6: Part 2, and it takes place in the civilized world of London.
Act II
It follows that Act II covers Bernard and Lenina's time in the Savage Reservation: Chapters 7, 8, and 9. It is during Act II that we have our protagonist switch from Bernard to John.
Act III
The last act begins right around the time John is puking his guts out in response to the new world. The transition from Act II is his shifting opinion of the nature of society, from admiration to disgust.