The film starts by referencing Siegfried Sassoon’s open letter dated July 1917, protesting the conduct and insincerities of the First World War. The letter has been published in The Times and has received much attention in England. With the string-pulling and guidance of Robert Graves, a fellow poet and friend of Sassoon, the army agrees to send Sassoon to Craiglockhart War Hospital, a psychiatric facility in Scotland, rather than court martialling him. At Craiglockhart, Sassoon meets Dr. William Rivers, a former anthropologist turned psychiatrist who encourages his patients to express their war memories so that they can “heal their nerves”. There is no clear main character in this film, but there is more focus on several of the characters; notably, Billy Prior, Siegfried Sassoon and Dr. Rivers himself. A very important secondary character, Wilfred Owen, is linked to Sassoon’s storyline.
Prior, at first an unsympathetic character, presents a challenge to Dr Rivers, who gradually discovers that the loss of Prior’s power of speech is the result of a traumatic experience in the trenches. Prior spends his spare time going into the town in search of sex, eventually beginning a relationship with Sarah, a munitions worker. He too has anti-war sentiments and refers to incidents that have caused him to distrust the military authorities.
Sassoon’s letter is read in the House of Commons and is dismissed, as he is considered mentally unstable. He begins to become friends with another patient in the hospital, Wilfred Owen. Owen aspires to be a poet as well and he greatly respects Sassoon’s work; Sassoon agrees to help Owen with his poetry. Meanwhile, Doctor Rivers has taken a leave of absence from the hospital and visits Dr. Lewis Yealland’s practice in London. Dr. Yealland treats his patients not like traumatised human beings, but like mere machines, which need to be repaired as quickly as possible. Rivers sits in on one of Yealland’s electroshock therapy sessions on a private named Callan, who, like Prior, has lost his speech. Rivers is at first repulsed by the brutality of the treatment, but back in Craiglockhart, he starts to question his own method of therapy, since his patients take long to recover and sometimes actually suffer under the expression of their traumas.
Sassoon has also come to a very important decision. Although he still disagrees with the brutality and suffering of the war, he decides to return to France in order to care for his men. Both Graves and Rivers are pleased with this decision. During the Review Board’s evaluation of Sassoon, Rivers is torn apart by conflicting feelings. On the one hand, Sassoon’s views have not changed in the slightest, and as such he still fulfills the qualification of mental illness that landed him at Craiglockhart. On the other hand Sassoon did not truly qualify as mentally ill in the first place, and he strongly wishes to return to the war. When his opinion is needed, he qualifies Sassoon as being fit, and thereby qualified to return to war.
In the meantime, Prior escapes a return to the Front when he goes before the medical board and is found to have a weak chest. The final scenes show Wilfred Owen’s death in France and Rivers’ despair on hearing of it.