The first volume of The Once and Future King, The Sword in the Stone, begins as the Wart, an innocent and wholesome boy living in twelfth-century England, is informed by his adoptive father, Sir Ector, that he must begin his education. While wandering in the Forest Sauvage after a night of adventure with King Pellinore (who hunts for the Questing Beast), the Wart chances upon the cottage of Merlyn, an old magician who "lives backward" through time and thus possesses the ability to know the future. Merlyn informs the Wart that he will become his tutor and accompanies him back to Sir Ector's Castle of the Forest Sauvage. After reviewing the wizard's references, Sir Ector hires the old magician. Kay, the Wart's older brother, becomes jealous over the Wart's fortune, as he does throughout the novel.
Merlyn's lessons consist of transforming the Wart into different kinds of animals. The boy's first transformation is into a perch, and while swimming in the castle's moat, he meets Mr. P., a ruthless tyrant who talks to him about power. At different points in the novel, the Wart becomes a hawk, an ant, an owl, a wild goose, and a badger: Each animal reveals to the Wart a different way of life, political philosophy, or attitude toward war. Merlyn also has his pupil witness a tilting match (or joust) between King Pellinore and Sir Grummore, where the two men reveal their absurd need to follow the rules of sportsmanlike combat.
Feeling sorry for Kay, the Wart asks Merlyn if he can transform his older brother into an animal as well; the magician explains that he cannot (since that is not what Merlyn was sent for). However, Merlyn does tell the Wart that he and his brother should follow a certain path into the Forest Sauvage, where they will surely find an adventure. The boys do just that and eventually meet Robin Wood, the famous outlaw (often called, in error, "Robin Hood") who explains to the boys that a band of fairies, the Oldest Ones of All led by the witch Morgan Le Fay, have kidnapped his companion, Friar Tuck, and the Dog Boy, one of Sir Ector's servants. The boys agree to help Robin and his men storm the Castle Chariot (a fortress made entirely of food) where the captives are being held. After rescuing the men, Kay kills the griffin (a creature with an eagle's head and wings and a lion's body) that guards the castle.
Sir Ector receives a letter from Uther Pendragon, the King of England, informing Sir Ector that the King is sending Sir William Twyti, his royal huntsman, to the Forest Sauvage to kill some wild boars. Sir Ector is expected to receive and care for Twyti and his retinue during his stay. When Twyti arrives, Sir Ector gives a great Christmas feast in which songs are sung and Sir Ector delivers a warm speech. At the boar hunt, the prey is killed but Beaumont, Twyti's favorite hound, is paralyzed from the waist down. While Twyti holds him in his arms, Robin kills the dog to free it from pain. King Pellinore then happens upon the Questing Beast, which has become sick with longing for her once-fanatic hunter, and King Pellinore vows to resume the chase.
Six years pass. Kay prepares for his impending knighthood while the Wart continues his education. (The Wart will become Kay's squire after he is knighted.) King Pellinore informs Sir Ector, Sir Grummore, and Kay that Uther Pendragon has died without an heir, and to remedy this politically chaotic situation, a sword has appeared outside a church in London, running through an anvil and into a stone. The inscription on the sword's pommel reads, "Whoso Pulleth Out This Sword of the Stone and Anvil, is Rightwise King Born of All England." A tournament is announced on New Year's Day to give all able men in England a chance to remove the sword. Kay convinces his father that they should attend, and he agrees. The Wart then enters, upset at Merlyn's announcement that he will no longer be tutoring him. Merlyn does assure the boy, however, that they will meet again.
During the tournament, Kay arrives at the tilting fields and realizes he has forgotten his sword; He orders the Wart to return to their inn and retrieve it. Finding the inn locked and nobody there, the Wart searches for a replacement. He eventually spies the sword in the stone and, and after a short struggle and the guidance of some animal friends, removes it, not realizing the significance of such an action. He returns to the tournament and tells Kay where he found the sword; Kay then lies to Sir Ector and claims that he pulled it from the stone. When they all go back to the stone, however, Kay admits his falsehood and, with his father, falls prostrate before the Wart, hailing him as King. The Wart, confused and embarrassed, bursts into tears.
Eventually, the Wart overcomes his awkwardness with his new title and is given a great party for his coronation. All of the characters offer him gifts. Merlyn reappears and tells the Wart that his real father was Uther Pendragon. He further informs the Wart that, in the future, it will be his "glorious doom" to "take up the burden" of his nobility. After promising to stay with the Wart for a long time, Merlyn addresses him as King Arthur.