Rivers sits in a corner, watching as Yealland brings Callan—a soldier who has lived through almost every major battle in the war—in to begin his treatment. Except for Rivers, the two men are entirely alone in the room. Yealland closes all the blinds so it is completely dark, aside from the glow of the battery and the reflection off Yealland's white coat and glasses.
Callan is brought in and strapped down to the chair. Rivers senses that the patient is frightened. Yealland attaches electrodes to the back of Callan's throat and continues to shock him repeatedly until he can make a sound. He shocks Callan for over three hours. When it seems like the patient is falling asleep, Yealland makes him get up and walk around the room. Callan tries to escape, but he cannot get out the door. It is locked from the inside, and only Yealland has the key. Callan considers attacking Yealland, but decides against it, and he submits to more electric shocks. To Rivers, the shocks seem extremely strong.
Yealland removes the electrodes from the back of Callan's throat and applies them to the sides of his neck. Callan seems to honestly be trying to speak, but only the sound "ah" comes out. Eventually, after many more shocks, Callan is stammering words. Yealland makes him feel completely powerless, repeating to him: "You must speak, but I shall not listen to anything you have to say." Finally, Callan is able to speak in full sentences. Yealland lets Callan up, and Callan smiles at him. Yealland does not like Callan smile, so he makes him sit down again, applies electrodes to his lips, and shocks him. When he gets up again, Callan no longer smiles. At the very end of the treatment, Callan must thank Yealland for curing him.
Rivers returns home that night, but finds himself unable to work or sleep. He is haunted by the images of Yealland and Callan, and the awful memories of the scenes of electro-shock therapy. He dreams of himself holding an electrode over a helpless patient's mouth, but the electrode turns into a horse's bit, and though he tries to push it into the mouth, it will not fit. Rivers wakes up screaming.
Rivers tries to understand his nightmare. He feels that the patient in his dream might have been Prior. Like Callan, he was mute upon arrival and seemed smugly satisfied with it. Rivers had used the tongue depressor on Prior in a moment of irritation with him. This makes Rivers wonder whether there is any real distinction between himself and Yealland. Both of them are in the business of controlling people, of making young men back into soldiers when they no longer want to be soldiers. Although Rivers prides himself on his more humane treatments, he nevertheless feels guilty for occasional pain he has inflicted on his patients.
Rivers realizes that what he witnessed that day was not the curing of the patient, but the silencing of one. By forcing Callan to speak, Yealland was forcing him to break, to relinquish his protest. It is then that Rivers realizes that the patient in his dream was not Prior, but Sassoon. Rivers confronts his guilt over breaking Sassoon, using his influence to force him to abandon his protest.
Rivers goes to consult with Head about his feelings of guilt. Head is surprised, and he reassures Rivers that there is no one person who could be more different from Yealland. Head tells Rivers that no matter how much influence he thinks he has, Sassoon has a mind of his own and it was Sassoon's personal decision to go back to the war.
Rivers talks about his trip to the Solomon Islands as an anthropologist. The natives asked him about his way of living; when he told them some of his culture's practices, they laughed uncontrollably because the things he described were so bizarre to them. At that moment Rivers felt incredible freedom, realizing that there was no measure of culture, of morality, or of right and wrong. It was "the Great White God dethroned."
Rivers returns to Craiglockhart for the monthly Board meeting and finds the hospital much quieter than when he left it. Anderson has worsened; he has not lost his fear of blood and he appears to be growing increasingly dependent on psychological help. Rivers visits Sassoon and finds him in his room in the same position as when he left him. Sassoon has finished a book, entitled Counter Attack, and has tried to remain out of trouble. He has also received some effusive letters from Owen, and he has begun to suspect that Owen had a crush on him. Rivers brings Sassoon the news that it seems very likely, if all goes well in the Board meeting the next day, that Sassoon will be sent back to France.
At the Board meeting the next day, they decide to give Anderson a desk job in the War Office rather than force him to go back to civilian medicine right away. When Sassoon comes in, he salutes the three officers and gives very satisfactory answers to all their questions. He admits that he has not changed his views on the war in the least; he is still very strongly opposed to it. Nevertheless, he considers it his duty to return to France and continue to serve his country. The Board agrees, and sends him back to active military duty.
Rivers says goodbye to Sassoon and reflects on the irony of the situation, that he himself should be changed so much by someone whom it is his job to change. Rivers worries about Sassoon, believing that he is going back to the war with the intention of being killed, that underneath his desire to watch over his men, Sassoon has a strong desire for death. Though Rivers has always respected authority, it is his turn to rebel. Morally, he must challenge a government that would kill an entire generation.
In these final chapters, Rivers is drawn to reflect on the existence of certainty and control. All of his life he has lived by the rules society has dictated for him, bound by abstract concepts such as duty and honor. Though these rules of gentlemanly behavior always worked for him before, in the chaos and horror of war he is forced to bring them into question. The military and the government in time of war work by basing their decisions upon certainties: "it is wrong to shirk your war duty," "it is right to sacrifice for your country," and "it is wrong to undermine the cause by publicly disagreeing with the war." By insisting upon these certainties, the government controls the minds and actions of men. Much like Dr. Yealland, it uses its arms and branches—such as Rivers—to force submission and conformity.
Yet, after years of working with shell-shock cases at Craiglockhart, Rivers is forced to confront the true definition of madness. Is Sassoon mad for rebelling unwisely against the war? Or is Rivers the one who is truly crazy? He is the one who works to heal people with the sole aim of returning "cured" patients to service, where they only face death. Rivers is forced to consider that he might be mad for working to defend a state that could sacrifice its own children so mercilessly. The questions of right and wrong, duty and honor are mingled and confused in the chaos of war.