The story starts in the last years of the rule of king Uther Pendragon. The first part, The Sword in the Stone, chronicles Arthur's upbringing by his foster father Sir Ector, his rivalry and friendship with his foster brother Kay, and his initial training by Merlin, a wizard who lives through time backwards. Merlin, knowing the boy's destiny, teaches Arthur (known as "Wart") what it means to be a good king by turning him into various kinds of animals: fish, hawk, ant, goose, and badger. Each of the transformations is meant to teach Wart a lesson, which will prepare him for his future life.
Merlin instills in Arthur the concept that the only justifiable reason for war is to prevent another from going to war, and that contemporary human governments and powerful people exemplify the worst aspects of the rule of Might.
Neither the ant nor goose episodes were in the original Sword in the Stone when it was published as a stand-alone book. The original novel also contains a battle between Merlin and sorceress Madam Mim that was not included in The Once and Future King but was included in the Disney film.
In part two, 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, Merlin 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 Guinevere's forbidden love, the means they go through to hide their affair from the King (although he already knows of it from Merlin), and its effect on Elaine, Lancelot's sometime lover and the mother of his son Galahad.
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 Guinevere, Sir Lancelot, and the entire ideal kingdom of Camelot.
The book begins as a quite light-hearted account of the young Arthur's adventures, 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 changes tone until Ill-Made Knight becomes more meditative and The Candle in the Wind finds Arthur brooding over death and his legacy.