After King Arthur hears of the crimes of King Royns of North Wales, he calls the knights from all his lands to a general council at Camelot. When the council is assembled, there comes a damsel sent by the Lady Lyle of Avilon. Under her mantle, the damsel has a sword which is fixed in its scabbard and cannot be drawn out except by a knight completely pure of heart. It is her curse that she must wear the cumbersome sword and scabbard everywhere she goes. Arthur and all his knights try to draw the sword and fail. At last, a prisoner among them, Sir Balyn, newly released from the dungeon and shabbily dressed, asks that he be allowed to try, for though he has been accused of a crime, and though his apparel is humble, he believes himself worthy. He draws the sword out easily, making some of the other knights fiercely jealous, and he says he will keep it, even though it is a fated sword: it will murder his dearest friend. Arthur apologizes for misjudging and mistreating Balyn, a man proved so noble.
Balyn accepts the apology and prepares to leave the court. Before he has left, the Lady of the Lake arrives and demands the gift Arthur promised her when she gave him his sword. She wants either Sir Balyn's head or that of the damsel who gave him his sword. Arthur refuses, and Balyn, recognizing her as his mother's murderer, hacks off her head. Arthur is outraged as a visitor to the court she was in his safekeeping and says he will never forgive Balyn for this murder. Balyn leaves and resolves to kill the tyrant King Royns and thus win back Arthur's respect. Sir Launceor of Ireland, one of Arthur's knights whom Balyn's success has humiliated, asks permission to ride after Balyn and avenge the Lady of the Lake; Arthur, still angry, grants it.
Now Merlin arrives and reveals the history of the sword. The damsel who brought it once loved a knight who was slain by her brother. She took the lover's sword to the Lady of Avilon and asked her help. With witchly whimsy, the Lady of Avilon sealed the sword in the scabbard so that only the best and hardiest man in the kingdom would be able to draw it, and with it he would slay not her brother but his own. Merlin reprimands the damsel for bringing the sword here, knowing its curse.
Launceor of Ireland now sets out after Balyn. They fight, Balyn kills him by accident, and Launceor's lady takes his sword and brandishes it. Balyn tries to get the sword away from her but cannot without hurting her wrist. When he lets go, she kills herself with it. Balyn is shocked and grieved at this needless waste and hurries away. He meets his beloved brother Balan, tells him all that has happened, and agrees to let him join the hunt for King Royns. A great king comes by and asks who killed Launceor. Balyn tells him and the king predicts that Launceor's relatives will want vengeance. The king reveals that he is King Mark (a vicious double-dealer later in Malory), then encamps to bury the bodies as befits their station.
Merlin appears and tells King Mark that in this burial spot will one day be fought the greatest battle ever fought between two knights who dearly love one another — Launcelot du Lake and Tristram. Merlin will not tell Mark his name, but on the day Tristram is taken with his lady, then Merlin will give both his name and news King Mark will be sorry to hear. Then Merlin tells Balyn that because he let this lady die (though he could not prevent it) he is fated to strike "the Dolorous Stroke," a stroke more terrible than any but that which killed Christ. Balyn does not believe him. If he thought he were capable of such an act, Balyn says, he would kill himself on the spot. Merlin vanishes. Balyn and Balan take their leave of King Mark, Balyn identifying himself as The Knight with the Two Swords.
As the brothers ride on, Merlin appears in disguise and shows them where King Royns is. They kill his attendants, wound him badly, and send him to Arthur. Merlin reveals that the knight who captured Royns was Balyn, and again Arthur repents his hasty judgment of Balyn. Merlin says that Royns' brother Nero will come with a great host tomorrow for vengeance and Arthur prepares. Then, to give Arthur's army a chance, Merlin goes to Nero's ally, King Lot, and holds him with tales of prophecy until it is too late for him to help Nero. Balan and Balyn join Arthur's forces and fight brilliantly. A messenger tells Lot what has happened — Arthur has easily destroyed Nero and his forces and is now in a position to destroy King Lot. Lot is furious at Merlin's trick but will not accept terms from Arthur because of Arthur's seduction of Lot's wife.
As for Merlin, he is grieved that Lot must die, but he has known from the start that in this battle it must either be Lot or King Arthur. Sir Pellanor, the Knight of the Questing Beast, kills Lot, for which deed he will later be killed himself by Lot's son Gawain. Lot's forces flee and Arthur buries Lot, Nero, and the twelve kings who supported them. Merlin adorns the tombs with symbolic figures and tells Arthur more of what is to come.
He warns that he will not remain with Arthur long and that Arthur must guard his magical scabbard carefully, for the woman he trusts most will steal it from him. Arthur gives the scabbard to his sister Morgan le Fay for safekeeping, and she gives it to her lover Accolon. Merlin tells, too, of the battle of Salisbury, against Mordred.
After these revelations, Arthur lies sick and heavy with thought. A moaning knight rides by, and Arthur sends Balyn to bring him back. Balyn brings the knight, parting him from his lady, and as they approach Arthur's pavilion the moaning knight is murdered by a knight named Garlon, who is invisible. Balyn returns to the dead knight's lady, taking over his quest. Another knight joins them and is similarly slain by the invisible knight. They bury him and on his stone appears a prophecy of Gawain's vengeance on Pellanor.
Balyn and the damsel ride on, come to a castle, and Balyn enters. A gate drops, separating him from his lady, and men set on her as if to kill her. Balyn climbs a tower, leaps a wall to help her, and learns that in this castle every passing maiden is bled, for a dishful of some maiden's blood will cure the sick lady of the castle. Balyn bleeds the maiden himself, without harming her, but the blood is not pure enough-only that of Percival's sister will do, and she will die giving it.
Now Balyn is directed to King Pellam's castle, where he will find Garlon. Balyn kills Garlon before Pellam's eyes, and Pellam fights to avenge his brother. Losing his sword, Balyn takes a marvelous spear and strikes with that. The castle falls to the earth, all but Pellam and Balyn are killed, and the land goes to waste. Balyn has struck the Dolorous Stroke. Merlin rouses Balyn and tells him that Pellam will not be whole until Galahad heals him in the Grail Quest, for this is the country where Joseph of Aramathy brought "parte of the bloode of oure Lorde . . . " and the spear is the one that killed Christ.
Balyn parts from Merlin and rides grieving through the Wasteland. When he has passed out of it he comes upon a knight who grieves because his lady has missed her assignation. Balyn helps the knight find the lady — sleeping in an ugly knight's arms. In a rage, the jealous lover strikes off their heads as they sleep, then mourns worse than before, for he has killed what he loved best; then he kills himself. Again Balyn is to blame.
Miserable, Balyn rides on and comes to a castle where he is told he must fight a knight who guards an island of ladies. One of the knights of the castle lends Balyn a shield better than his own. Balyn fights the guardian of the island in order to pass, and because Balyn does not have his usual shield, the island guardian — Balyn's brother — does not recognize him. They wound each other mortally, but before he dies, Balyn learns that if he had won and lived, it would have been little better. Because Balan killed the earlier keeper of the island, he has been bound to take over his position, which would now have fallen to Balyn.
The brothers are buried in a single grave, and around the tomb, partly with the doomed brothers' relics — their swords and scabbards — Merlin sets up events of the future. He sets a new pommel on Balyn's cursed sword, and now no man can handle it but Launcelot or Galahad, and with this sword Launcelot will kill his dearest friend, Gawain. He leaves Balyn's scabbard for Galahad to find, and he puts Balyn's sword into a floating stone to be attained by Galahad.
"The Knight with the Two Swords" is an ingenious and complex development of the two closing motifs of the "Merlin." In the first place, "The Knight with the Two Swords" focuses on the ironic destinal forces which Merlin can only in part control. Trusting in God and in "adventure" — or Fortune — Balyn takes the sword that is rightfully his. He knows himself to be pure in heart — his winning of the sword proves it — and so he cannot believe he will kill the man he loves best. Neither can he believe, later, that he will strike the Dolorous Stroke. He does both. Moreover, every pure and good cause he undertakes results in catastrophe: in his self-defense against Launceor of Ireland he causes the death of the knight, and in his wish not to hurt the wrist of Launceor's lady, he allows her the chance to commit suicide. So it is with all he does. Even in striking the Dolorous Stroke he acts without guilt, unaware of the consequences. His limitation is simply that he is mortal — non-omniscient — and the limitation is underscored after every mistake either by the appearance of magic writing or by Merlin's prediction, through his greater foreknowledge, of what later catastrophes will come in these same places, perhaps as direct or indirect results of Balyn's actions.
The second motif developed from the end of the "Merlin" is that of vengeance. Every detail Malory has brought together from his widely scattered sources involves vengeance (usually as family revenge) or its ironic inversion, intentional or accidental betrayal by a member of the family or by a lover. In each case, the vengeance or betrayal of love is unpredictable for ordinary men. The damsel who carried her lover's sword to the Lady of Avilon could not know that, for mysterious reasons of her own, the witch would turn it into an instrument of monstrous harm. Arthur could not know, in sending Launceor after Balyn, that in acting on his own outrage (coupled with Launceor's jealousy) he would trigger far greater wrongs. Nor could Balyn know that in pursuing King Royns, he would rouse King Nero.
It is through Arthur's war with Lot, Nero, and the eleven kings that Arthur's kingdom is unified, but this unity is grounded on the same principle of violence that operates in personal feuds, weaving an ever more intricate web of revenge and betrayal and debt. Once begun, the process cannot be turned back. While men exact payment of an eye for an eye, both the ministers of Fate (the Lady of the Lake and, sometimes, Merlin) and also the design of Providence strike out at petty acts of violence with terrifying force. If the purest and most just of men can be destroyed in this process — that is, Balyn — the fault must lie in the chivalric code itself, if anywhere. And yet it is the code which, with Merlin's help, establishes Arthur's rule of order. Without civilized order and the redressing of wrongs, the world could have no defense against tyrants like Royns, outlaws like Garlon, men of desperate need, such its the blood thieves of the sick lady's castle, or cruel faithlessness (such as Morgan). The chivalric code is founded, in short, on deadly paradox.