1917. The Great War grinds on. Second Lieutenant Siegfried Sassoon is on convalescence leave but is refusing to return to the war, protesting against it. A report of this protest reaches Sassoon's superiors. They think he is mad. Sassoon's protest is that a war of defence and liberation has become one of aggression and conquest. He believes the fighting men are being deceived.

Sassoon recalls discussing his protest with a handful of similarly minded intellectuals, including Bertrand Russell, Hugh Massingham, Lady Morrell and Lytton Strachey. They encourage him to follow his conscience if he can withstand the punishment that will inevitably follow.

Sassoon is recalled to his base camp for an interview with a superior. While waiting for this interview he reads reports of soldiers executed for cowardice. The adjutant proves amiable, telling Sassoon that the Colonel is concerned about him and is willing to overlook the matter, but asks him to reconsider his protest. Sassoon refuses. He denies being a pacifist or conscientious objector, but explains that when the war began it was clear what the war was for, but not anymore. He believes the war will achieve nothing but slaughter.

In defence of the war, the adjutant rants excitedly about the menace of Germany. He sends Sassoon to a hotel to await the return of the Colonel. Sassoon resolves to stick to his statement of protest and wait for it to be read out in parliament by the backbench MP, Lees-Smith.

At the hotel, Sassoon recalls his time in France. He remembers horseplay with fellow officers in a commandeered house and later revisiting the house gutted by battle.

At the hotel, he is frustrated to find no letter from Lees-Smith to say that his statement has been read. He sends a copy of the newspaper report to his friend Geoffrey Cromlech. Sassoon recalls a night-time excursion with Cromlech into No Man's Land to retrieve greatcoats from the dead to pass on to his freezing men back in the trenches.

The adjutant visits Sassoon at the hotel. Sassoon recalls his fellow officers playing piano and singing, and Cromlech insisting that they are fighting not for freedom but for Mesopotamian oil. Sassoon asserts that they are all being exploited. Cromlech criticises Sassoon, suggesting he attempts to align himself with the aristocracy in an attempt to hide his mixed blood.

The adjutant tells Sassoon that he has arranged a medical board for him attend the next day. Later, Sassoon destroys the order to attend the board. He worries about being imprisoned for his protest, anticipating a five-year sentence. He visits a music hall but disapproves of the jingoistic pro-war entertainment, and the soldiers on leave who enjoy it.

Sassoon receives a letter from Lees-Smith indicating that the opportunity to read his statement is imminent.

Sassoon is visited by his CO, Colonel Jones-Williams. He is angry at Sassoon's refusal to attend the medical board. He warns him that he risks court martial and disgrace, and asks him to reconsider his refusal.

In an excited state, Sassoon inconveniences hotel patrons with his thoughts on the difference between 18th century determinism and 19th century humanism.

In the hotel restaurant, Sassoon is approached by two old comrades. They irritate him with their tales of the front and he walks off.

Sassoon throws the ribbon of his Military Cross decoration into the sea. Cromlech, also on convalescence leave, visits him at the hotel. He urges him to drop his protest. He believes he will achieve nothing. The pair argue.

Down by the sea, Cromlech accuses Sassoon of making his protest for personal reasons, because he wants to be a martyr. Cromlech tells him that he knows he will not be court-martialled. Instead, the military authorities will declare him shell-shocked and lock him away. Cromlech encourages Sassoon to agree to the medical board. Finally, Sassoon relents.

As he leaves his hotel, Sassoon reasons that he can now only attempt to be passed fit for service and return to the front.

Later, Lees-Smith reads Sassoon's statement in parliament. The opposition replies that Sassoon has been examined by a medical board, diagnosed with a nervous breakdown and sent to a hospital for shell-shocked officers.