The book centers on Paul Bäumer, a German soldier on the Western Front during World War I. At the start of the book, Paul lives with his parents and sister in a charming German village. He attends school, where the patriotic speeches of his teacher Kantorek lead the whole class to volunteer for the Imperial German Army shortly after the start of The Great War. Bäumer arrives at the Western Front with his friends and schoolmates (Leer, Müller, Kropp, Kemmerich and a number of other characters). There, they meet Stanislaus Katczinsky, an older soldier nicknamed Kat, who becomes Paul's mentor.
While fighting at the front, Bäumer and his comrades engage in frequent battles and endure the treacherous and filthy conditions of trench warfare. The battles fought here have no names and seem to have little overall significance, except for the impending possibility of injury or death. Only meager pieces of land are gained, which are often lost again later. Remarque often refers to the living soldiers as old and dead, emotionally drained and shaken. "We are not youth any longer. We don't want to take the world by storm. We are fleeing from ourselves, from our life. We were eighteen and had begun to love life and the world; and we had to shoot it to pieces."
Paul visits home, and the contrast with civilian life highlights the cost of the war on his psyche. The town has not changed since he went off to war, but he has: he finds that he does "not belong here anymore, it is a foreign world". He feels disconnected from most of the townspeople. His father asks him "stupid and distressing" questions about his war experiences, not understanding "that a man cannot talk of such things". An old schoolmaster lectures him about strategy and advancing to Paris while insisting that Paul and his friends know only their "own little sector" of the war but nothing of the big picture.
Indeed, the only person he remains connected to is his dying mother, with whom he shares a tender, yet restrained relationship. The night before he is to return from leave, he stays up with her, exchanging small expressions of love and concern for each other. He thinks to himself, "Ah! Mother, Mother! How can it be that I must part from you? Here I sit and there you are dying; we have so much to say, and we shall never say it." In the end, he concludes that he "ought never to have come [home] on leave".
Paul is glad to return and reunite with his comrades. Soon after, he volunteers to go on a patrol and kills a man in hand-to-hand combat for the first time. He watches the man die slowly in agony for hours. He is remorseful and devastated, asking for forgiveness from the man's corpse. He later confesses to Kat and Albert, who try to comfort him and reassure him that it is only part of the war. Afterwards, they are sent on what Paul calls a "good job". They must guard a supply depot in a village that was evacuated due to being shelled too heavily. During this time, the men are able to adequately feed themselves, unlike the near-starvation conditions in the German trenches. In addition, the men enjoy themselves while living off the spoils from the village and officers' luxuries from the supply depot (such as fine cigars). While evacuating the villagers (enemy civilians), Paul and Albert are taken by surprise by artillery fired at the civilian convoy and are wounded by a shell. On the train back home, Albert takes a turn for the worse and cannot complete the journey, and instead is sent off the train to recuperate in a Catholic hospital. By a combination of bartering and manipulation, Paul manages to stay together with Albert. Albert eventually has his leg amputated, while Paul is deemed fit for service and returned to the front.
By now, the war is nearing its end and the German Army is retreating. In despair, Paul watches as his friends fall one by one. Kat's death is the last straw that finally causes Paul to lose his will to live. In the final chapter, he comments that peace is coming soon, but he does not see the future as bright and shining with hope. Paul feels that he has no aims left in life and that their generation will be different and misunderstood.
In October 1918, Paul is finally killed on a remarkably peaceful day. The situation report from the frontline states a simple phrase: "All quiet on the Western Front." Paul's corpse displays a calm expression on its face, "as though almost glad the end had come."