Winston Smith is lying on a camp bed, where he has been for many days, being tortured almost constantly. O'Brien oversees Winston's "treatment." Finally, O'Brien personally takes over, torturing Winston when he does not give the correct answer to the questions O'Brien asks, many of which have to do with memory and objective truth.
O'Brien finally answers Winston's primary question, the question that has haunted him throughout the story: the why of the Party's behavior. Winston also learns that he is thought to be insane, and O'Brien, who acts strangely like Winston's friend, says that he will cure him. O'Brien allows Winston to ask him whatever he wants, and O'Brien seems to answer honestly.
In Chapter 3, Winston enters the second stage of his "reintegration," understanding. Here, in his conversations with O'Brien, Winston learns about the Party's ideology and debates with O'Brien about the spirit of Man. Winston is able to look at himself in the mirror, a ruined, crushed human being, for which O'Brien mocks him. Winston discovers that Julia has betrayed him, but he has not yet betrayed Julia. Finally, O'Brien tells Winston what he knew all along — that he will eventually be shot — but is ambiguous about when.
Winston's horrors and fear are brought to light in these chapters: He is betrayed by Julia and O'Brien, he is tortured and ruined, and every hope he had for a future without the Party is destroyed. Winston learns that Goldstein's book was written partially by O'Brien and that Big Brother exists just as the Party exists, eternal and omnipotent.
These chapters function much like the chapters of "the book," which Winston read earlier in the story; both serve to answer unanswered questions about the Party and its ideology. However, these chapters are more revealing and shed light on many of the things Winston has wondered about throughout the novel. He always understood how the Party wielded its power, but he never understood why; O'Brien explains to him that the Party seeks power solely for the sake of power, ironically like the bird or the prole woman singing just to sing, as Julia had observed earlier.
O'Brien tries to make Winston understand and employ the concept of doublethink; doing so will be Winston's only salvation, but Winston finds mustering the mental strength to do so difficult. This unwillingness to use doublethink has been Winston's downfall from the beginning and ultimately proves to be his breaking point.
Again the theme of the importance of objective truth returns. Here, Winston takes the position that memory and objective truth must win out over falsehood because the Party cannot destroy memory. O'Brien is set on proving Winston wrong in this case. O'Brien tells Winston that the Party is far superior to Nazi Germany or the Russian Communists because, unlike those other regimes whose enemies were eventually turned into martyrs, the Party refuses to let a stray thought get through. Controlling all thought is the Party's power, a power that will remain timeless.
Here Orwell takes totalitarianism a step further — into the mind. The equation that Winston writes in his diary, 2 + 2 = 4, comes back to haunt him; it is the one objective truth that Winston cannot give up. The equation is the sticking-point between Winston and O'Brien and ultimately becomes the proof of Winston's reintegration. If Winston can believe that 2 + 2 = 5, then the Party has gotten inside of him.
O'Brien knows about every "criminal" activity that Winston has engaged in to this point — even something as "minor" as Winston's memory of the photograph of Aaronson, Jones, and Rutherford upon which he had been basing much of his evidence that the Party was deliberately changing history. Even without the physical photograph, the image still exists in Winston's memory, and O'Brien uses this image as an example of Winston's inability to want to change for the better — the better of the Party.
O'Brien tells Winston that Julia has betrayed him, but there is no evidence in this chapter to prove that it is true. Winston has not betrayed Julia, and that fact is the only thing that keeps him from being "reintegrated" — the only thing keeping him human.