Emperor Lucius of Rome sends messengers to Arthur's court demanding tribute. Though Arthur is furious, he controls himself and takes council with his knights. Sir Cador is delighted with the possibility of honorable war; the King of Wales vows vengeance on the Viscount of Rome, who once treated him shamefully; the young Launcelot du Lake eagerly offers his aid; and the remaining knights also pledge their full support. Arthur gives the envoys of Rome their answer and they leave. They warn Lucius of Arthur's might, but Lucius attacks nevertheless, supported by giants and Saracens. He takes the lands Arthur won from King Claudas and moves toward Normandy. Arthur leaves his kingdom in the hands of two noblemen and Guinevere, and, though his wife swoons from grief, prepares to embark. If he dies, Constantine, Cador's son, is to succeed him.
On his ship, Arthur dreams of a battle between a dragon and a huge bear. According to his dream interpreter, the dragon represents Arthur himself; the bear is either some tyrant or a giant he will destroy. As soon as he lands at Normandy he hears of a giant who torments the land, murders women and children, and has recently stolen a duchess, wife to Arthur's cousin. Arthur calls Kay and Bedivere and tells them to arm themselves; they will go with him on a pilgrimage to St. Michael's Mount, "where mervayles are shewed."
They ride through a beautiful countryside full of birds, then alight. Arthur says he will seek the "saint" alone. He finds the grave of the duchess, beside it an old woman who warns him that the giant has no respect for treaties. He will accept nothing from Arthur but Guinevere. Arthur fights the giant, kills him, and jokes more with Bedivere and Kay on this "saint" he has found. He gives away the giant's treasure, attributes the victory to God, and moves on.
Word comes that Lucius is fairly close, and Arthur sends King Bors and Gawain to warn Lucius that he must withdraw. A knight at Lucius' court, Sir Gains, mocks Gawain, and Gawain — quick-tempered and vengeful as ever — cuts off Gaius' head. The two knights flee; the Romans pursue; Gawain and Bors are forced to turn and fight. They drive the foremost Romans back, and as the Romans withdraw, Round Table knights burst from ambush and destroy Romans on every side. In this fight Sir Bors and Sir Berell are captured. Gawain, furious at this indignity, breaks through the Roman ranks and, with a younger knight's help, rescues his friends.
Throughout the battle Gawain fights nobly, taking highborn prisoners and enduring painful wounds. When he and the others return to Arthur's hall, Arthur greets him eagerly and says he would give him the prisoners' heads if he thought it would help his wounds. Gawain graciously dismisses the half-offer. Then Arthur sends the prisoners to Paris for incarceration, with Sir Cador and Launcelot at the head of the expedition.
Launcelot and Cador meet an ambush of Romans. The British force is small and weak, but the older knights make knights of their squires and fight heroically. In the end, largely through Launcelot's incredible fighting ability, the Romans are overwhelmed, nearly all of them being killed. The British return to Arthur and report the victory and their own minor losses. Arthur condemns the battle as foolhardy, but Launcelot insists that to turn back even when overmatched would be shameful, and his fellow knights support him.
The Romans who make it back to Lucius beg him to drop his hopeless war against Arthur. Lucius scoffs and launches a new attack. In this battle, the Welsh king fulfills his vow to destroy the Viscount of Rome, Launcelot steals Lucius' banner, and all the other great knights — Cador, Kay, Gawain, Bors, Pellas, Marhault, and others — fight better than ever before "syn God made the worlde." Sir Kay and Sir Bedivere are wounded, almost mortally, and now Arthur fights more fiercely than ever. Meanwhile Gawain and Launcelot fight splendidly, side by side. Arthur now forbids any taking of prisoners: in vengeance for Sir Kay, whom he thinks to be dying, all the Romans and their allies are to be slaughtered. Afterward, Arthur buries his dead, and Kay and Bedivere recover. Arthur then sends the bodies of the Romans home as his "tribute." If this tribute is insufficient, he will give more of the same when he comes to Rome, he says.
Now Arthur moves southward, recapturing the lands taken by the Romans. While laying siege to a city in Tuscany, he sends Sir Florens, Gawain, and two other knights on a foraging expedition, with supportive forces. While the others graze their horses in a meadow, Gawain rides out to scout the countryside and meets a Saracen knight, with whom he fights. Each severely wounds the other before the Saracen submits. He tells Gawain that his name is Priamus and that because of his excessive pride his father sent him to this battle to humble him. When he asks Gawain's name, Gawain at first claims he is a mere yeoman, then admits the truth, and Priamus is thankful that he has lost to a man so worthy.
They return to the meadow where the horses are feeding, and Priamus heals both their wounds with the water of Paradise. Then Arthur's knights prepare for battle with the enemy force Priamus says is close at hand. The fight takes place, Priamus and his men join Arthur's side, and Arthur's men win. Back at the city walls, Arthur christens Priamus and makes him one of his own vassals. The siege on the city is successful, and Arthur promises mercy to all but the recreant duke. He orders his men not to molest the city's women.
Then Arthur moves on to be crowned in Rome, distributes land and wealth, and at last, at the request of his nobles — they have been too long parted from their wives, they say — he turns back toward England, where Guinevere and the other wives joyfully welcome him and all his troops.
In "Arthur and King Lucius" Malory has transformed the alliterative Morte Arthure, one of the earliest full-scale tragedies in English, into a tale of the Round Table at its happiest and, in some ways, most noble. For the overweening pride of the Arthur in his source, Malory substitutes a just and wise king; and the poem's tragic conclusion, Arthur's return to fight Mordred, lover to Guinevere, Malory drops for a joyful reunion of faithful husbands and wives. In the source, Launcelot is a minor figure, Gawain central. Malory elevates Launcelot, retains the dignity of Gawain, making only this distinction between them: Gawain's rash action in murdering Gains is left unjustified, while the action of Launcelot and Cador on the road to Paris, labeled as rash in the source, is carefully rationalized here.
But the most important change is Malory's introduction of the marriage theme. The giant of St. Michael's Mountain wants Arthur's beard in the source. In Malory we are told twice that he wants Guinevere. The murdered duchess in this episode is changed from a cousin of Arthur to the wife of a cousin. And so, throughout Malory's version, all references to marriage in the source are retained and new references are introduced. At the same time, other love relationships are introduced and developed here. It is in this tale that we first see Gawain and Launcelot as devoted friends, fighting in one another's behalf Arthur's love for Kay, present in the source, is retained and made central.
On another level, true kingship is obliquely identified with marital love, false kingship with rape. (It is a medieval commonplace that the relationship of the king and the state is "marriage.") The St. Michael's giant, as we have said, is emphatically identified with the rape of wives (the duchess; Guinevere).
In the source, Lucius' army includes German giants; but Malory sets them in sharper relief by suppressing details which distract attention from the giants. Lucius' destruction of fair lands involves the murder of women and children, while Arthur's capture of the Tuscan city, here as in the source, rules out harm to women, children, or anyone else other than the duke. The bear in Arthur's dream refers simultaneously (as in the source) to the tyrant Lucius and to the giant on the mountain. One further change Malory made was his introduction of parallels between Arthur and Henry V. (For discussion of this point, see Vinaver, Works, 111, 1361-62.)