The story is told from the perspective of the fictional character Thomas "Tommo" Peaceful, a young man from a rural village in England who fights in the First World War. The novel switches between Tommo's past and present: in a short section at the beginning of every chapter, Tommo describes his thoughts and feelings during a single night in a hay barn near the Somme. Each chapter indicates the progress of time, with titles such as "Five Past Ten" and "Twenty To Eleven", gradually counting down to the morning of his brother Charlie's execution by firing squad. The majority of the novel covers Tommo's memories of his life.
He recalls being a young boy before the First World War. He discusses his love for Molly, a girl he met on his first day at school, and his relationship with his older brother Charlie. Early in the story, Tommo and his father go woodcutting together, leading to his father's death while he saves Tommo from a falling tree; Tommo keeps the incident a secret from everyone, blaming himself for what happened. Tommo, Charlie and Molly grow up together; their mischievous adventures include braving their nasty great-aunt, defying a Colonel, skinny-dipping, and being the first people in their village to see an aeroplane.
Charlie, Molly and later Tommo all find jobs on the local estate or in the village. Charlie and Molly become closer as they are both older than Tommo, causing Tommo to feel left out. Later, it is revealed that Molly had become pregnant with Charlie's baby. She is thrown out of her house and moves in with the Peacefuls. Tommo is heartbroken after the couple rush to get married. A short time later, Charlie is forced to enlist in the British Army and is deployed to France to fight in the First World War. Tommo lies about his age to join his brother. The rest of the story describes the brothers' experiences of the war: their confrontations with Sergeant "Horrible" Hanley, near-misses during the fighting on the front line, and Charlie's continued protection of Tommo.
During a charge of the German lines, Charlie disobeys a direct order from Sergeant Hanley and stays with Tommo while he is injured in no-man's-land. As a result, Charlie is accused of cowardice, for which he is court-martialled and sentenced to death. On the night before his brother's execution, Tommo reveals to Charlie his guilt for their father's death; Charlie reassures him that his father's death was not his fault. Tommo learns that Sergeant Hanley has been killed, but this only provides small consolation. Charlie is marched before the firing squad and dies happily, singing his favourite childhood song, "Oranges and Lemons". The novel ends with Tommo preparing for the Battle of the Somme.
