The protagonist, George Sherston, comes from a privileged background and, before the war, had been shocked even at the thought that a fox might be killed during a fox hunt. He joins the army filled with idealism and a desire to serve his country. However, as he is exposed to the horrors of war, his mind and viewpoint are changed. A particular turning point comes when Dick Tiltwood—a gentle Welshman who, in many ways, represents innocence and purity—is killed. This moves Sherston to protest against the war which he now sees with clear eyes as something villainous and immoral; he has moved from the innocence of the pre-war age to the experienced cynicism of post-war England, which, like Sherston, can never be the same again.

Despite the suffering of the trenches, there is a light to be found in the close bonds established between Sherston and his friends Cromlech and Tiltwood. The three men develop very close ties which, in many ways, are intensified by the circumstances. It is his friendship with Cromlech which saves Sherston from being court-martialed for his objections to the war.

Of course, the subject of the book is war, and as such, Sassoon seeks to present war in all its horrible reality. Consequently, the theme of war and its brutality permeates the novel; Sherston approaches war as an idealistic soldier but encounters many horrific sights, culminating in the atrocities of the Battle of the Somme.