Uther Pendragon, King of England, is Arthur's father. He conceives Arthur out of wedlock (although later he marries her and Arthur is legitimized) with another man's wife. Igraine, the wife of the Duke of Cornwall, is deceived by sorcery to believe that Uther is her husband. The child who results is taken by the sorcerer responsible for the deception, a mysterious magician named Merlin, to be raised by another family. Uther dies soon after. This leaves Arthur King of England, but the baby's whereabouts are unknown and the kingdom is in disarray.
During this time of unrest, the clergy and barons meet to discuss the succession, for the existence of Arthur is not widely known. When the meeting takes place at "the greatest church in London" (15) a wondrous sword is found standing in a stone. On the stone is written "Whoso pulleth out this sword of the stone and anvil, is rightwise king born of all England" (16) None of the barons can move the sword, and things are at an impasse.
Sir Ector, a good knight, raises Arthur. Ector's son, Kay, is older than Arthur, and bosses him around somewhat. While on the way to a tournament, Ector discovers he has forgotten his sword. He sends Arthur back for it, and, finding the door to the house locked and everyone away at the tournament, Arthur pulls the sword easily from the stone for his foster-brother. He brings it to Kay, who tries briefly to take credit for it. Sir Ector questions him, and finds tha Arthur is the one who performed the feat. After much dispute among the barons, Arthur is crowned.
At the great feast of Pentecost in Wales, the hubands of Arthur's three half-sisters (the daughters of Uther) come and make war on him disputing the suceesion. They are each kings of other parts of Britain, Kings Lot, Nantres, and Uriens, and they think to take England for themselves through force and by right of Uther's daughters' claims to the throne. They are unsuccessful, and Arthur shows himself valorous in battle and a great commander. He is advised by two foreign Kings, Ban and Bor.
Arthur becomes entangled with the wife of King Lot, and they conceive a child. Arthur does not know that this woman is his half-sister (she turns up later as Morgan Le Fay, the child is Mordred). Arthur meets and marries the lovely Guinevere. He is told by Merlin of the existence of Mordred, and tries to kill all the noble children born on Mordred's birthday. He is unsuccessful.
Arthur fights a battle with Sir Pellanor, a rogue knight, and loses his sword. He obtains one, with Merlin's help, from the Lady of the Lake -- a mysterious presence whose arm rises from the water holding the magical sword Excalibur.
Sir Balyn, an untrustworthy knight, kills the Lady of the Lake. This angers Arthur, and to try to assuage this Balyn misguidedly kills Sir Launcelor. King Lot attacks again, and again Arthur is victorious.
Guinevere brought a huge table, seating one hundred and fifty knights, as her dowry. Thus the Knights of the Round Table were created.
Source: Mallory, Thomas. Le Morte D'Arthur. Middlesex, England: Penguin, 1987.