Le Morte d'Arthur tells the story of King Arthur and his Knights at the Round Table. Arthur, who is son of King Uther Pendragon but was raised by another family, takes his rightful place as king when, as a boy, he is able to pull the sword called Excalibur from the stone. Although he rules wisely and is counseled by Merlin the magician, Arthur makes enemies of other kings and is often at war.
When Arthur marries Genevere, her father gives Arthur the Round Table, at which 150 men can sit. Genevere, who is often present at the convening of the Round Table, acts as a moral compass for the knights, rewarding knights who behave well and chastising those who choose poorly. Malory specifically relates the stories of Sir Gawain, Sir Tor, and Sir Pellanor as a means of introducing the concept of chivalry.
Arthur is nearly betrayed by his sister Morgan le Fay, but he is helped by Nineve, a sorceress who learned her magic powers from Merlin before killing him. Arthur then fights the Romans when Emperor Lucius of Rome demands that Arthur bow to him. Although the war requires several battles, Arthur and his knights win and return to Guinevere and the other wives. Soon after, Launcelot establishes himself as the greatest knight in all the world by his virtue, loyalty, and bravery. At the same time, Sir Gareth, Gawain's brother, proves valiant in his adventures.
Tristam (also known as Tristan), who is son of King Melyodas de Lyones and the sister of King Mark of Cornwall, is then introduced, and his adventures unfold. He kills Sir Marhault to free his uncle from a debt owed to King Angwyssh of Ireland, and then falls in love with Isode (also known as Isolde), Angwyssh's daughter. Isode marries Tristam's uncle Mark, but Tristam and Isode remain lovers. Tristam is exiled by Mark, which means he can no longer use his true identity; thus, he fights as The Knight with the Black Shield. Tristam duels and beats many of Arthur's knights, but is eventually thrown in prison and becomes ill. He escapes and eventually meets and fights Launcelot in a duel predicted by Merlin. They become the best of friends.
Launcelot, who is in love with and completely loyal to Guinevere, rides one day in search of adventure. He kills a dragon, sees the Grail, and is tricked into lying with Pellas' daughter Elayne, with whom he has a son, Galahad. Guinevere, upon hearing of the affair, has Launcelot banished from court; Launcelot then wanders from place to place in his grief. Elayne, through her father, heals Launcelot through the Grail, and he eventually returns joyously to Camelot and the Round Table.
Launcelot introduces his son, Galahad, to the court, and Galahad takes the Sege Perilous, the seat at the Round Table that no knight has been worthy enough to fill. Galahad also draws the sword from the floating stone, establishing him as the best knight in the world, but also accepting the sword's curse — that it will later cause a grievous wound.
Most of the knights then set out separately on Grail Quest. During the Quest, Launcelot, Percival, and Bors experience deep religious conversion, while Ector and Gawain are told by a hermit that they are not pure enough to achieve the Grail Quest. Galahad, Percival, and Bors meet up and continue the Grail Quest, but they are briefly parted. Launcelot and Galahad continue to the Grail at Castle Corbenic, where Launcelot is shown to be unworthy of the Quest. When Sir Evelake dies after his embrace with Galahad, Galahad is identified as the knight who will achieve the Grail Quest. Galahad is made a king who dies shortly thereafter, while Percival becomes a hermit. Bors returns to King Arthur's court.
Launcelot also returns to the court and continues his love for Guinevere. After a series of trials, Guinevere is convinced of Launcelot's love for her. Although Arthur knows of the affair and overlooks it, he is prompted by Aggravain and Mordred (Arthur's son by Lot's wife) to take action; Guinevere is sentenced to be burned at the stake. Launcelot rescues her and takes her to his castle, Joyous Gard, but in the battle, Launcelot kills Gareth and Gaheris, who are at the execution but are unarmed. Launcelot returns Guinevere to Arthur, but Launcelot is banished, along with his followers. Gawain swears vengeance for the death of his brothers and insists that Arthur attack Launcelot. Arthur agrees, but while Arthur and Gawain are away, Mordred makes himself King of England, claims Guinevere as his wife, and attacks Arthur's army. Gawain is mortally wounded and warns Arthur in a dream not to continue the battle. Through a misunderstanding, however, the battle continues; Arthur kills Mordred but is mortally wounded by him, as Merlin has prophesied.
Launcelot and Guinevere both die of illness soon after, and Constantine becomes king. The Round Table is disbursed.