Dr. W.H.R. Rivers, a psychiatrist at a mental hospital in Craiglockhart, Scotland, silently reads a letter written by Siegfried Sassoon in July 1917. Sassoon's declaration, a "willful defiance of military authority," clearly and logically states his decision to stop fighting as a soldier in World War I. Sassoon believes that the purpose of the war has changed; what was once a war of liberation and defense has become one of aggression. He cites the suffering of the troops, the political insincerities, and the "callous complacence" of those at home as reasons for his protest. Rivers notices that Sassoon has signed the letter "S. Sassoon," conveniently leaving out his first name, Siegfried.
Bryce and Rivers discuss the possibility of Sassoon becoming a patient at the Craiglockhart hospital for "shell-shocked" soldiers. Rivers expresses his reservations, doubting that Sassoon is really shell-shocked and hesitant to shelter a "coward" who just wants to escape the fighting. Rivers is also concerned that bringing Sassoon to Craiglockhart will bring bad publicity to the hospital. Eventually, however, Bryce talks Rivers into taking Sassoon as a patient.
Sassoon, on a train on his way to the mental hospital, thinks of a meeting he had with his friend Robert Graves a week earlier. In that meeting, Graves told Sassoon he had received his declaration protesting the war. Graves advises Sassoon to give up his cause and not to make a martyr of himself. Though Graves believes that the war has become unjust, he urges Sassoon to go to the mental hospital rather than allow himself to be court-martialed. Graves has pulled some strings; by showing the military board some of Sassoon's letters in which he writes of hallucinations of corpses in Piccadilly Circus in London. These letters have been enough to convince the board to put Sassoon in a mental hospital. But Sassoon had hoped to be court-martialed so that he could bring more attention to his pacifist cause. Graves convinces him that he must go to the mental hospital.
Rivers is at the window when Sassoon arrives at the door to the mental hospital. He looks at the report and thinks it strange that Sassoon should have thrown away his medal for saving life. During a raid of the trenches, Sassoon had remained under fire and had risked his life to bring in all the dead and wounded from the field. Rivers watches as Sassoon overcomes his fear and walks into the gloomy building.
Now that Sassoon has arrived at the hospital, he meets with Rivers to talk over tea. Having tea gives Rivers a chance to evaluate the mental state of his patient. The two men have a pleasant conversation. Sassoon tells Rivers that the medical board has been rigged; the decision to send him to a mental hospital was made before the Board even evaluated him. It is easier for the Board to pass his letter off as madness rather than admit that it represents a valid charge against the government. Sassoon admits that he does not have any religious reason for opposing the war; he is merely horrified at the senseless brutality of it.
Sassoon tells Rivers about some of his hallucinations about corpses, and about some of the things he was asked to do in the war. He admits that he no longer hates the Germans. Rather, he hates the complacent civilians at home who allow the war to go on, completely blind to the atrocities it entails. Sassoon asks Rivers if he thinks that he is mad; Rivers replies no, of course not. Nevertheless, Rivers informs Sassoon that he cannot be impartial; as a psychiatrist in the mental hospital, it is his duty to convince Sassoon to return to the war.