The title comes from the supposed inscription of the marker over King Arthur's grave: HIC IACET ARTORIVS REX QVONDAM REXQVE FVTVRVS — "Here lies Arthur, the once and future king."
The book is divided into four parts:
A final part called The Book of Merlyn was published separately (ISBN 0-292-70769-X) following White's death. It chronicles Arthur's final lessons from Merlyn before his death, although some parts of it were incorporated into the final editions of the previous books.
In fact, Merlyn instills in Arthur the concept that the only justifiable reason for war is to prevent another from going to war then, and that contemporary human governments and powerful people exemplify the worst aspects of the rule of Might.
In The Queen of Air and Darkness, White sets the stage for Arthur's demise by introducing the Orkney clan and detailing Arthur's seduction by their mother, his half-sister Morgause. While the young king suppresses initial rebellions, Merlyn leads him to envision a means of harnessing potentially destructive Might for the cause of Right: the Round Table.
The third part, The Ill-Made Knight, shifts focus from King Arthur to the story of Sir Lancelot and Queen Guenever's forbidden love and its effect on Elaine, the mother of Lancelot's son, and the King.
The Candle in the Wind unites these narrative threads by telling how Mordred's hatred of his father and Agravaine's hatred of Sir Lancelot caused the eventual downfall of King Arthur, Queen Guenever, Sir Lancelot, and the entire ideal kingdom of Camelot.
The book begins as a quite light-hearted account of the young Arthur's adventures, Merlyn's incompetence at magic, and King Pellinore's interminable search for the Questing Beast. Parts of The Sword in the Stone read almost as a parody of the traditional Arthurian legend by virtue of White's prose style, which relies heavily on anachronisms. However, the tale gradually becomes darker until Ill-Made Knight loses much of the original humor and The Candle in the Wind is mirthless.
It is also interesting to note that White allows Thomas Malory to have a cameo appearance towards the end of the final book. Also of note is White's treatment of historical characters and kings as mythological within this world that he creates. In addition, due to his living backwards, Merlyn makes many anachronistic allusions to events in more recent times; of note are references to the Second World War, telegraphs, tanks, and "an Austrian who … plunged the civilized world into misery and chaos" (i.e. Hitler).
Underscoring the story of Arthur's life, from his youth and education to the end of his reign, is a well thought out commentary on how mankind should govern itself, written in the context of the Second World War. The political standpoints are totalitarianism, communism, anarchy, and socialism.
When Arthur first ascends to the throne, the country is ruled by what he calls Fort Mayne, or the rule of the strongest. The barons and nobles ride around the countryside doing whatever they wish--being unpleasant, exploitative, and sometimes murderous. Despite the ongoing question of whether humanity is naturally evil, through most of the book King Arthur is optimistic that there is a means to curb humanity's tendency toward violence and cruelty. The latter three parts of the book show the progression of his search for a solution. His first solution to the rule of power is to crush it with power ("Might is Right"). As a young king, he conquers rival barons in a war in which Arthur dispenses with gentlemanly protocols so as to force the barons to experience the horrors of war firsthand. However, this is clearly not a permanent solution, but merely perpetuates the problem.
His next move is to channel power into something worthy. He reinvents Chivalry, and forms the Round Table, making it a goal for his knights to use their Might to rescue maidens and right wrongs ("Might for Right"). However, this solution does not last for long. Once all the wrongs are righted, and England settles into a golden period of peace and lawfulness, the knights grow bored, and things at court start to go badly. Pettiness and squabbling arise, and society stagnates. This is what Merlyn calls "Games-Mania": the knights become caught up in Jousting and Tourneys, to the point that vicious rivalries are established, especially the Orkney-Lancelot one. A better solution is needed.
Arthur's next move is to seek the solution from outside the mundane world. He sends his knights on a quest for the Holy Grail — aiming their power toward God instead of toward worldly things ("Might for God"). This, however, is a failure, too, because any knight who achieves the quest is perfect, and thus no longer suitable to live in an imperfect world. The other knights who fail are for a time positively affected by the quest (Sir Lancelot in particular), but it does not take long for them to fall back into their old ways. In addition, many knights who fail the quest (Gawaine) feel humiliated by Lancelot and Galahad, and many good knights end up dying in the quest for the Grail.
Arthur's final solution as king is to formalise power: he reinvents Civil Law ("Right is Right"). Instead of power being wielded by the knights, it now belongs to the state. An example of this would be the replacing of trial-by-battle with trial by jury. This solution comes back to bite Arthur when the affair between Guinevere and Launcelot is exposed: adhering to his new law means that he must punish his beloved wife and his best friend, by banishing Lancelot and burning Guinevere. However, he knows that Lancelot will rescue her, and Lancelot does indeed end up rescuing Guinevere and they escape to his castle together. However, in the process he unintentionally kills the unarmed Gaheris and Gareth.
Almost everyone considered Gareth the "best" or most "knightly" of the Orkneys; he was knighted by Lancelot, and his brother Gawaine loved him. When Lancelot kills Gareth and Gaheris while they are unarmed during the rescue of Guinevere, not recognising them in his fury, Gawaine flies into a rage and Arthur into deep depression. Gawaine tells Arthur he has no choice but to go to war with Lancelot so Gawaine can extract vengeance.
The book ends with Arthur, weary and aged, in his field pavilion on the eve of the final battle between his knights and Mordred's Thrashers. He reflects upon where he has gone wrong, and whether humans can ever learn to renounce violence. Before going forth, Arthur charges a young page (Malory) with keeping alive his legend and his ideals until a better day.
This is where The Book of Merlyn fits in: Arthur is taken to Merlyn's cave, where he meets many of his old friends from The Sword in the Stone — animals with whom he has spent time. He then spends some time as an ant, and as a goose, experiencing the structure of their societies. The ant is a fiercely territorial animal, with a rigidly structured life. The goose, on the other hand, is free, without any boundaries or borders, flying where it wants. Arthur spends an idyllic few days as a goose, before he is dragged back to Merlyn's cave. He realises that boundaries, which don't actually exist, but are purely mental constructs in human minds, are the real cause of the strife in the world, and that humanity should do away with them if he wants to achieve a successful and peaceful society.