Le morte d’Arthur begins with the story of King Arthur of Camelot’s birth. King Uther needs to find an heir to his throne, and he has an eye on Igraine, the wife of the Duke of Cornwall, when they come to visit the court. Together with the wizard Merlin, he hatches a plan to lay siege to the Duke’s court while Igraine is at another castle. The Duke is killed in the siege, and Uther (whom Merlin has made to look like the Duke) sneaks into the castle. Uther manages to sleep with Igraine, who only later finds out that her husband has died. In exchange for his help, Merlin asks Uther to hand over the son that he will conceive with Igraine. Uther and Igraine marry, as Igraine is convinced this will be best for the kingdom, and she gives birth to a boy: Arthur. Merlin sends him to be raised by a knight, Sir Ector, alongside Ector’s son Kay.
After Uther’s death, the kingdom is in a precarious state, left without a king. Merlin, sharing his plan with the Archbishop of Canterbury, has an enchanted sword, Excalibur, placed in a stone by the great church in London. All the lords gather and each attempts to draw the sword out, but none proves able. Sir Ector comes to London for a New Year’s Day tournament and brings Kay and Arthur. Kay has just been made knight, but he’s forgotten his sword at home, so he asks Arthur to retrieve it for him. Hurrying home, Arthur sees the sword in the stone and easily pulls it out. But when he comes back, Ector recognizes it, and they return to the stone so that Arthur can prove he managed to pull it out himself.
Everyone present agrees that Arthur is the proper king of England, and he is soon crowned king. But it doesn’t take long for other kings to become skeptical about such a young king of England, and wonder if they can take advantage of Arthur’s inexperience to gain some lands for themselves. Merlin warns Arthur of the threat, and suggests Arthur ask for help from two kings, King Ban and King Bors, and then return the favor by fighting in their wars. Together the kings come close to routing King Lot and his allies, but they stop once Merlin counsels Arthur to quit while they’re ahead. Then Arthur’s men defeat the enemies of Ban and Bors. While abroad Arthur first meets Guenever, who will later become his queen, but he also meets and sleeps with Margawse, wife of Lot and father to Gawaine, Gareth, Agravaine, and Gaheris, though Arthur he does not realize she is his half-sister. She will give birth to a son by Arthur, Mordred, who is fated to kill his father and overturn the kingdom. Merlin counsels Arthur to send all the children born in May (the month of Mordred’s birth) onto a ship that will be shipwrecked—he does so, but Mordred is the only one to survive, while the senseless deaths of the other children make many in the kingdom angry with Arthur.
Arthur does what he can to bolster his kingdom’s strength by establishing the fellowship of the Round Table, in which dozens of knights take up a place around a huge physical table and pledge to seek honorable quests, grant mercy to opponents, and defend ladies wherever they might find them. Arthur will need this fellowship, especially since Merlin fades from the story—he falls in love with Nimue, the Lady of the Lake, who learns sorcery from him and then, once she grows tired of him and afraid of his power, entraps him in a cave forever. Arthur also has to deal with the trickery of Morgan le Fay, his sister and a sorceress, who is always trying to either kill him or trick him into doing her will.
Although Arthur has defeated Lot and his men, he soon has to face more threats when Emperor Lucius of Rome demands tribute from him. Believing this to be a shameful demand that would threaten his sovereignty, Arthur refuses and takes his knights onto the European mainland, where they have a number of adventures, including the slaying of a dangerous giant, before defeating the Roman armies and returning in triumph. Launcelot du Lake, one of Arthur’s knights, performs particularly well in Rome. He accomplishes a number of adventures back in England as well, and proves himself as the world’s greatest knight.
At Pentecost every year, the knights of the Round Table reunite to share stories of their exploits and regroup before returning to their quests and adventures. One year, an unknown squire arrives to court to try to prove himself. He’s dressed shabbily and no one knows who he is, so Arthur has him help out in the kitchen. Kay makes fun of the boy, calling him “Beaumains” or “Fair-hands,” though the other knights rebuke Kay for his teasing. When a damsel, Linet, comes to court asking for a knight to rescue her sister Lionesse from the Knight of the Red Launds, Beaumains asks to fulfill this quest. He rides out, pursued by Launcelot, and asks Launcelot to be made knight. Once he shares his true identity with Launcelot—Beaumains is actually Gareth of Orkney, brother of Gawaine—Launcelot agrees. But no one else knows Gareth’s true identity, and Linet continues to harangue him for his shabby clothes and lack of noble identity, even when Gareth strikes down many knights and proves to have great prowess. Finally Gareth defeats the Knight of the Red Launds, and “wins” Lionesse. Only after Gareth proves himself at the Pentecost jousting, however, does Linet stop preventing him from sleeping with Lionesse, and the couple is married.
For a large part of the book, the narration then switches to a knight outside of King Arthur’s court—Tristram, son of King Meliodas. Tristram’s stepmother tries to poison him out of jealousy, and while Tristram forgives her—establishing his knightly honor—his father sends him out of the country. When he grows up, he proves himself by defending King Mark of Cornwall against King Anguish of Ireland, and by fighting a knight of the Round Table named Marhaus. Tristram wins but is wounded, and Anguish’s family takes care of him, not realizing who he is. He falls in love with the king’s daughter, La Beale Isoud. Before long, Isoud’s mother learns who Tristram really is, and while Anguish understands that Tristram had to defend Mark on account of his honor, he sadly sends Tristram from his court. Tristram stays at King Mark’s court, but after they both fall in love with the same lady—Isoud, who loves Tristram—Mark grows wildly jealous and begins to plot Tristram’s downfall. Mark decides to order Tristram to fight and “win” La Beale Isoud so that Mark can marry her himself. As a good king’s subject, Tristram does so, though he and Isoud continue to love each other. One knight of Mark’s eventually tells the king of their mutual infatuation, and he locks Isoud away. Tristram escapes to King Howel’s land, where he briefly falls in love with (and marries) another woman, Isoud La Blanche Mains. He ends up leaving this Isoud and meets up with a knight of Arthur’s court, Lamorak de Galis, to defeat a tyrant together.
Meanwhile, back at Arthur’s court, another unknown knight, Breunor le Noire (La Cote Male Taile) comes to court anonymously, proves himself through his knight’s prowess, and is made a knight of the Round Table. A number of the Round Table knights meet and joust with Tristram, who still resists being made a knight of the Round Table, since he thinks he’s not worthy enough—besides, Launcelot, who has learned of Tristram’s infidelity to La Beale Isoud, is angry with him. Tristram sneaks back to Mark’s court, but once again is found out. After a fight with Isoud he flees into the forest, where he seems to lose his sanity for a time. He is eventually brought back to court as a madman, but eventually Isoud recognizes him—once his identity is revealed, Mark banishes Tristram from court. Finally Tristram, after proving himself on the battlefield in Arthur’s tournaments, is made a knight of the Round Table. Mark hears of this and is jealous, so he sneaks into England—though not before killing two of his knights who refuse to help him kill Tristram. A few knights of Arthur’s court then come across Mark (though he hides his identity). Finally one, Dinadan, learns who he is, and Mark is brought to Arthur. Arthur has him swear not to plot against Tristram any longer, and Mark does so, and brings Tristram back to court with him. Meanwhile, Gawaine and his brothers lose out to Lamorak at a tournament. Jealous, they kill their mother Margawse—Lamorak’s lover—and eventually Lamorak too. Back at Mark’s court, Mark is forced to ask Tristram to help him in defeating enemies. Mark’s brother also performs very well, so Mark grows jealous and kills him. But the brother’s wife and son, Alisander, escape.
Mark continues to plot to destroy Tristram, but Launcelot is back on Tristram’s side, and together they manage to imprison Mark for a time. Free to have adventures on his own, Tristram joins with several other knights, including Gareth, Palomides, and Dinadan, and they pursue a number of quests. But Palomides is also in love with La Beale Isoud, and after Tristram gains glory at a tournament, Palomides grows nearly sick with jealousy. The two prepare to fight for their honor, but Tristram is wounded and so cannot battle Palomides. Meanwhile, Launcelot—who maintains an affair with Queen Guenever, to whom he is loyal for the entire book—is tricked into sleeping with Elaine of Corbin, who gives birth to Galahad, a knight fated to surpass even his father in greatness.
Later, Galahad arrives at court without sharing his identity, but it soon becomes clear that he is holier than any of the other knights when he pulls yet another sword out of an enchanted stone. With great fanfare, his arrival means the start of the quest for the Sangreal or Holy Grail—a vessel that is able to grant limitless food and drink, and also reveal spiritual mysteries, though only to the one who is holy enough. Arthur is sorry to see so many of his knights leave on the quest, since he knows most of them will not be worthy enough to achieve it. Galahad’s adventures on the quest are detailed first, as he frees several castles from evil knights and defends several damsels. Percivale, another knight on the quest, tries to find Galahad, but instead has to confront his own quests, which involve temptations from and battles with the devil in various guises. Launcelot too is confronted with such temptations, and while he battles well as always, he is told by a number of figures that because of his earthly sins—like sleeping with Guenever and embracing earthly pride—he will never be able to achieve the Holy Grail. Sir Bors also faces a number of spiritual tests, first having to choose between saving a lady and saving his brother Lionel. He chooses the lady, thus showing his knightly honor, but Lionel grows furious and, possessed by the devil, tries to kill his brother. However, Bors manages to flee and join Percivale. They meet up with Galahad, where they have many adventures together. They meet Percivale’s sister on an enchanted ship, where there is a sword and scabbard invested with holiness because of their connection to a Biblical figure and early guardian of the Holy Grail, Joseph of Arimathea. Galahad turns out to be fated to take this sword and scabbard. Eventually, the three knights reach the Castle of the Maimed King, Pellam, where Galahad heals the king, thus fulfilling a prophecy. Another king throws them into prison, but the Holy Grail ensures that they are kept fed and healthy, and when the king dies Galahad is crowned king of the land. After some time, Galahad finally is granted the right to see the spiritual mysteries of the Holy Grail, and is raised to heaven. Percivale becomes a holy man and dies not long after, while Bors returns to Arthur’s court, where many of the original knights of the Round Table have died.
Back at court, Launcelot soon forgets the vow he had made to become a holier man, and resumes his affair with Guenever. Soon, however, one knight Pinel tries to poison Gawaine out of jealousy at Guenever’s feast, but accidentally kills a knight Patrise. Everyone thinks it is Guenever, but Launcelot defends her against Pinel. Several tournaments are held, and Launcelot proves himself mightily. At one point he stays with a lord Bernard and his daughter, Elaine le Blank, who falls in love with him, although Launcelot continues to be loyal to Guenever—Elaine eventually dies out of grief. After another great tournament, the kingdom is at peace for a time.
Soon, however, a knight named Meliagrance tries to take advantage of Launcelot’s absence from court to kidnap Guenever, whom he is in love with. Launcelot is captured and thrown into prison, but finally, thanks to a damsel’s intervention, he escapes and kills Meliagrance in battle. But another knight, Agravaine, is jealous of Launcelot and decides to plot against him. Launcelot’s affair with Guenever has always been an open secret at court to everyone but Arthur, and Agravaine finally shares the secret openly and proves it to Arthur by surrounding Guenever and Launcelot when they are in bed one night. Launcelot escapes, and Guenever is sentenced to death. Launcelot manages to ride in and rescue Guenever before she is burned at the stake, though in the mayhem he accidentally kills Gareth and Gaheris. Their brother Gawaine has always been loyal to Launcelot, but this is the last straw, and he vows to destroy Launcelot. Arthur sorrowfully agrees to fight against his best knight and friend. At one point Launcelot, who has taken Guenever to his tower, returns her to Arthur, who would happily end the civil war, but Gawaine refuses to be satisfied until Launcelot is killed. Launcelot flees to Benwick with some knights loyal to him, and Gawaine and Arthur, with their knights, depart to lay siege there. While they are away fighting, Mordred hears of Arthur’s death and crowns himself king, making Guenever his wife. Even after he finds out Arthur is still alive, he refuses to give up his position. Arthur and Gawaine are forced to return. Gawaine is mortally wounded and on his deathbed tells Arthur he forgives Launcelot and is sorry for ever starting the war. Arthur is told to sign a truce with Mordred until Launcelot can return to fight for him, but at the last minute an accident renders the truce null, and Arthur and Mordred mortally wound each other. Only then does Launcelot return, though it’s too late. Guenever retires to a nunnery, and many of the other knights become holy men or hermits, giving up a life of battle. A new king, Constantine, is crowned, ending the story of the Round Table.