A classic of 20th-century literature, The Once and Future King is based loosely on the epic Middle French poem Le Mort d'Arthur, written by Sir Thomas Malory in the 15th century. Malory's text chronicles the early life, career, and death of the legendary King Arthur, who supposedly lived and reigned during the 6th century although no physical evidence of him has survived. White takes liberties with the story, setting it in 14th-century England and reworking several of the main characters and events.
White's book is divided into four parts, the first of which is devoted to the education of Arthur as a young man. He is assisted by Merlyn, a wizard who experiences life and time backwards and who therefore has knowledge of the potential future. This first sub-novel, The Sword in the Stone, can be regarded as a bildungsroman. The second novel, first published as The Queen of Air and Darkness, introduces the primary antagonist of the series: Arthur's half-sister Morgause, whose sons Gawaine, Gaheris, Agravaine, and Gareth eventually become "the Orkney faction." The third novel, The Ill-Made Knight, focuses on Lancelot and his forbidden romance with Queen Guenever, and Candle in the Wind describes the events leading up to Arthur's fateful battle with Mordred, his son by Morgause.
The novels that comprise the complete book were written immediately after World War II, when European powers waged what is now regarded as a classic good-versus-evil battle between the Allies and the fascist Axis powers. On the other side of the globe, Japan was expanding its empire, having invaded China and also attacked Pearl Harbor, thereby attracting the enmity of the United States. During World War II, European society and culture was pounded into rubble with round after round of bombing. Even after the war, the rebuilding process was long and hard. Poland was hit the hardest of any European nation, losing about a tenth of its population over the course of the war, and as facts about Japanese experimental facilities and Nazi gas chambers became more and more widely known due to the highly publicized Nuremberg trials, people began to question whether there was any way to permanently prevent such odious things from happening in the future. Furthermore, the use of the atomic bomb on the cities of Hiroshima and Nagasaki showed how easily entire cities could be annihilated. Accordingly, White's text makes occasional references to contemporary political and military facts. He mentions, for example, "an Austrian" (Hitler) who had caused a great deal of trouble.
White's interpretation of Arthurian legend is unique, because even his antagonists are well-rounded characters who have extremely good reasons for behaving as they do. The Round Table experiment, by which Arthur hopes to use the knights for law enforcement, eventually fails because the strongest and bravest knights in the realm cannot stop being human. They retain their individual likes, hatreds, and preferences regardless of their roles in the realm, and advance their personal and family interests as they understand them. Many of the main characters have radically different ideas about what "right" and "wrong" look like, and about which things are most important.
White wrote at a time when various conflicting national interests were pulling countries in different directions, much as the individual characters at the Round Table come to be at odds with one another. Ultimately, morality is presented not in the context of some absolute Truth (the way Wart, as a child, imagined it would be) but as a highly personal, almost relativistic phenomenon. Likewise, war is regarded not as a noble and energizing activity, but as something to ideally be avoided or prevented. Yet, by the end of the novel, a final war is inevitable.