LAUNCELOT du Lake returns from Rome most honored of all Arthur's knights and therefore most prized by the queen. He decides to seek knightly adventures, presumably to win still greater renown. He rides out with his nephew Sir Lionel and in the heat of noon goes to sleep under an apple tree. Lionel, keeping watch, sees a powerful knight overwhelm and tie up three weaker knights. When Lionel tries to help them he too is bound and carried back to the castle of the wicked Tarquin, where all four captives are stripped naked, beaten with thorns, and thrown into a dungeon. Ector, who has followed Launcelot and Lionel, is also caught.
While Launcelot is asleep under the tree, Morgan le Fay and three other ladies find him and fall in love with him. Morgan returns him to her castle by enchantment, and there the four ladies demand that he choose one of them or die. Launcelot refuses to choose any lady, and is saved by a maiden in return for his promise to help her father, Sir Bagdemagus, in his tournament.
Launcelot rides through a forest, finds a pavilion, and lies down to rest. A knight comes, mistakes his sleeping form for that of his lady, and lies down beside him. They fight, and Launcelot wounds the knight. The expected lady arrives and asks Launcelot to use his influence to make her knight one of the Round Table. Launcelot agrees.
The next morning he goes to the abbey where he is to meet Sir Bagdemagus. He wins the tournament for Sir Bagdemagus, then leaves to hunt for his nephew.
A damsel guides him to Tarquin, asking that if he wins this fight that he come to the aid of maidens distressed by a knight in the forest. While Launcelot and Tarquin fight, Tarquin explains that he persecutes knights because he is after the slayer of his brother — Launcelot. Sir Launcelot kills him, releases Tarquin's latest prisoner, Sir Gaheris, and sends him to free the other prisoners; then Launcelot rides on. All the freed prisoners ride back to Camelot except Lionel, Ector, and Kay, who resolve to find Launcelot instead.
Launcelot, meanwhile, rides with the damsel, traps the thief and rapist who has been troubling maidens and kills him. As the damsel parts from him, she advises him to marry, but Launcelot explains that neither marriage nor love of a mistress is fitting for a knight, for one ties him down and the other can involve him in wrong causes.
In time he comes to Tentagil Castle, where Uther conceived Arthur on Lady Igrayne — a castle where maidens have been imprisoned by giants for seven years. He slays the giants and rides on. He sleeps wherever he can and eventually comes to a castle where he is well lodged. That night he sees three knights attack a fourth — Sir Kay — and he leaps to Kay's rescue. Afterward, while Kay sleeps, Launcelot takes Kay's shield and armor, leaving his own, and rides off. In Launcelot's armor, Kay can ride home in peace, since no one will come against Launcelot. Launcelot, in the armor of boastful and unpopular Kay, has fights on his hands. He overcomes and impishly teases Ector, Ywain, and Gawain, among others, unhorsing them and leaving them so that they have "much sorow to gete their horsis agayne."
Following a hunting dog through the forest, Launcelot comes to a dead knight and his grieving lady. He consoles the lady, then departs and soon meets a damsel who tells him that the other knight in the recent battle, the damsel's brother, cannot be healed until some knight can be found who will go into the Chapel Perilous, find there a sword and bloody cloth, and bring them back to clean the wound. Launcelot goes, meets giant knights dressed in black who mysteriously make way for him when he charges; he takes the cloth and sword, and by perfect loyalty manages to escape the elaborate trap which has been set for him. Had he proved unfaithful either to the knightly code or to his virtuous love for the queen, he would have died, and the sorceress who loves him would have embalmed him to keep him at her side. He heals the wounded knight.
On the road again, he comes to a lady who asks that he retrieve her falcon from an elm tree, for if the hawk gets away her lord will kill her. Launcelot takes off his armor and gets the hawk. While he is up there, naked and unarmed, the lady's husband appears; the falcon was a trick, and the husband is here to murder him. Launcelot breaks off a branch, fights with that, and kills his would-be assassin. Next Launcelot encounters a knight who is about to kill his wife from jealousy. Though Launcelot tries to prevent the murder, the husband succeeds. Launcelot sends him to Camelot, where Guinevere imposes his penance and sends him for further penance to the pope.
At the feast of Pentecost, all Launcelot's great deeds are made known and he is acknowledged the greatest knight in the world.
Whereas "Arthur and King Lucius" celebrates the chivalric ideal as it informs and supports the group, the tale of Launcelot celebrates the ideal as embodied in one man. Launcelot's encounter with Tarquin is an obvious contrast between the best of knights and one of the worst: Tarquin fights for personal vengeance and delight in cruelty, scorning the Order of Knighthood and all its laws; Launcelot fights in defense of the Order.
But from this point on, the tale is paradoxical. Forcing men to submit to Kay, then wearing Kay's armor, Launcelot seems to fight not for personal glory but for virtue's sake — the glory going, at least for the moment, to Kay. At the Chapel Perilous he proves his faithfulness; in other battles he proves his wit, his pluck, and his mercy. On the surface Launcelot's love of Guinevere is not a central concern in this tale; but one notices that while various characters speak of the rumor of Launcelot's love for Guinevere, Launcelot himself never admits to it. He in fact denies that he is in a position to love or that Guinevere is anything but faithful to Arthur.
Launcelot's expressed views on love and marriage contrast dramatically with those of Uther and the Arthur of the first tale, ironically recalled in this tale when Launcelot stumbles onto Tentagil Castle. When Launcelot hears that this is the castle where Arthur was adulterously conceived, Malory says cryptically, "'Well,' seyde sir Launcelot, 'I understonde to whom this castel longith.' And so he departed frome them and betaught hem unto God." (The Tentagil episode is not found in Malory's sources.)
The reason Launcelot is Guinevere's knight at the end of the tale is that he is "the best in the world": her judgment of him is, as always, the judgment of civilization. Within this tale it is not shown that Launcelot has been seeking from her anything more than this symbolic approval. Nevertheless, Launcelot's behavior, even his language, as well as his attention to Guinevere, is nobler and more polite than that of Arthur himself.
Malory does not develop the ironic and dangerous potential in all this; but the potential is clearly there. Malory alters his sources to make Launcelot send his prisoners to the queen rather than to Arthur, and he makes it clear that Launcelot's disguise as Kay actually fools no one. In the end, Kay has been made a fool of, and Launcelot has replaced him in Arthur's favor as well as the queen's.
In short, what appears on the surface to be an illustration of model knighthood is in fact, as Professor R. M. Lumiansky says, "a prelude to adultery," a central cause of the Round Table's fall. The final irony, which comes out more and more clearly as Le Morte d'Arthur progresses, is that it is chiefly Launcelot's need to prove himself to the lady he loves that makes him the great knight he is.