Merlin, furious at Uther's actions, nevertheless agrees to use his magic to help Uther sleep with Igrayne, on condition that Uther relinquishes to Merlin whatever results from his lust. Merlin summons the dragon who lies beneath the land, and Uther, transformed into Cornwall's double, rides into the castle across the sky on the dragon's breath. Uther immediately and forcefully takes Igrayne, stripping her naked and raping her, at the same time that the real Duke of Cornwall is killed attacking Uther's camp. Igrayne's young daughter, Morgana, senses her father's death and sees through Uther's disguise.
Igrayne bears a son, Arthur. Upon seeing the boy, Uther speaks of creating peace and staying with Igrayne, but Merlin arrives to take the boy, as agreed. Igrayne's despair is such that Uther pursues Merlin to retrieve Arthur, but is ambushed and, despite a stubborn and desperate defense and flight, is mortally wounded in the forest by knights still loyal to Cornwall. Before the ambushers can seize Excalibur, Uther thrusts it into a large stone, and Merlin exclaims that "he who draws the sword from the stone, he shall be king".
Years later, Sir Ector and his sons Kay and Arthur attend a jousting tournament to win a chance to try to pull Excalibur from the stone. The best knights in the land gather to compete, but none is able to draw the sword. Arthur, acting as Kay's squire, forgets Kay's sword in a tent and returns to retrieve it, only to discover that it has been stolen. While pursuing the thief, Arthur stumbles by the stone in which the Sword is embedded. On a whim, Arthur draws it from the stone. The crowd from the tournament gathers around him, and after putting it back Arthur easily draws the sword from the stone again. Merlin then appears, revealing to Arthur that he is the son of Uther and Igrayne and therefore the rightful king of the land.
Not all accept Arthur's kingship. As the knights argue, Arthur flees into the forest, but Merlin explains Arthur's destiny, telling the boy that he and the land are one. Overwhelmed, Arthur slips off into sleep. The next morning, Merlin tells him that his enemies are laying siege to the castle of one of Arthur's main supporters, Leondegrance. Rallying the other knights loyal to him, Arthur leads a counter-assault and repels the attackers. During the battle, he meets Leondegrance's daughter Guenevere. The battle ends when Arthur asks Sir Uriens to take Excalibur and use it to proclaim Arthur a knight. Uriens at first hesitates, tempted to take Excalibur for himself, but is apparently overwhelmed by the sword's magic and compelled to grant his request. This act by the boy-king, together with Arthur's courage in the battle, earns him recognition.
Later, Arthur encounters Lancelot, a brilliant and heretofore undefeated knight. In search of a king worthy of his sword, Lancelot will allow none to pass a bridge until he is defeated in single combat. Lancelot bests Arthur in a joust, but the enraged king refuses to accept this, and challenges Lancelot to a duel to the death. Lancelot has the upper hand, striking deftly and seemingly at will on the furious Arthur, until the king summons Excalibur's power. The indestructible blade strikes, and breaks through, Lancelot's armour and seemingly kills him, but the sword is broken in half. A devastated Arthur confesses to Merlin that in his rage he abused the sword's power to serve his own vanity. Arthur throws what is left of the sword into a body of water but, upon his words of contrition, the Lady of the Lake rises to deliver a restored Excalibur to the king. Realizing his error, Arthur quietly vows never to abuse the sword's power again. Lancelot then recovers and swears fealty to Arthur.
After a series of battles, Arthur unifies the land, and he creates the fellowship of the Round Table, and builds Camelot, his castle. Arthur marries Guenevere, but it is clear from their first meeting that Lancelot and Guenevere feel a powerful mutual attraction. Arthur's half-sister, Morgana, a budding sorceress, becomes apprenticed to Merlin, hoping to learn the Charm of Making from him. Meanwhile, Lancelot, the greatest of the knights, is often inexplicably absent from the Round Table, seeking refuge deep in the forest in order to brood. While meditating there one day, Lancelot encounters a peasant boy named Percival, who becomes his squire.
Lancelot's forbidden love for Guenevere is what keeps him away from Camelot. One evening, Sir Gawain, under the spell of Morgana, openly accuses them of adultery at the Round Table. Arthur decrees that Lancelot must fight Gawain in a duel to defend Guenevere's honour. In a nightmare duel with himself, Lancelot pierces his own side with his sword in order to purge himself of his love for Guenevere. Lancelot's wound makes him late for the duel and Arthur requests that one of his other knights defend the queen. The only one willing to do so is Percival, who is hastily knighted by Arthur so that he can fight for Guenevere. At that moment, Lancelot arrives, and manages to defeat Gawain despite his injury; Gawain withdraws his accusation. Lancelot collapses, close to death. Arthur implores Merlin to bring him back, whatever the cost.
Ultimately, Lancelot and Guenevere consummate their love, and Arthur finds them asleep and naked together in the forest. Meanwhile, Merlin attempts to trick Morgana who is showing signs of hostility to Arthur. Arthur is seen thrusting Excalibur toward the sleeping couple, and Merlin is impaled by the sword because of his magical link with the Dragon and the Earth. Morgana takes advantage of his weakness to trap him in crystal with the Charm of Making, then takes the form of Guenevere to trick Arthur into making love to her. She bears a son, Mordred.
On awakening, Lancelot sees Excalibur and flees in shame, knowing Arthur has seen him with Guenevere. Guenevere realizes what has happened and enters a nunnery. The land is stricken with famine and sickness, and a broken Arthur sends his knights on a quest for the Holy Grail. Years pass and many knights die on the quest, while others are bewitched by Morgana to serve her and her son.
In the end, Perceval witnesses Mordred viciously murdering Uryens, the very man who knighted Arthur all those years previous, and comes to realize that he is the only knight left to search for the Grail. Narrowly escaping death at the hands of Mordred and Morgana, he encounters Lancelot, no longer the handsome, athletic knight he once was but now a fat, bearded, ugly old man who is still plagued by madness from his betrayal of Arthur, leading a group of people through the dying land; Percival recognizes Lancelot and tries to reach out to him, but Lancelot accuses Percival and the other knights of having brought the misfortune on the people of Camelot and, with the help of the people he is leading, he pushes Percival into a river. Rising out of the river, Percival, having lost his armor, has a vision of the Grail and a mysterious figure who asks "who am I?" and "what is my secret?" After being mercilessly beaten and shoved into a river by followers of the now disillusioned Lancelot, Perceval realizes that the figure is King Arthur, and his secret is that he and the land are one. Answering the riddle, he attains the Grail. Arthur drinks from it and is revitalized.
Arthur and his few remaining knights ride to war against Mordred and Morgana. The barren land blooms with life as they pass, reborn with its King. He goes to Guenevere's convent, where they are reconciled. She returns Excalibur to him, having kept it safe since the day she fled.
Most of the land's nobles have rallied to Mordred and Morgana. Arthur calls to Merlin in despair and unwittingly awakens him from his enchanted slumber when he strikes a monolith with his armoured fist. Though still imprisoned in Morgana's crystal, Merlin appears to her in dream and tricks her into uttering the Charm of Making, creating a thick fog. Her magically-endowed youth dissolves. When she tries to embrace Mordred, he strangles her to death in disgust.
Arthur and his small army survive by hiding in the fog, but they are soon overwhelmed. Out of the fog Lancelot, wearing rusty, broken armor, joins the fray and turns the tide of the battle. Lancelot falls to the ground, finally having his reconciliation with Arthur. He still suffers from the old wound from fighting himself which has never healed. Arthur admits to his beloved champion that he was the very best knight of the Round Table, and Lancelot succumbs to his death. Still hoping to defeat Arthur and become king, Mordred runs Arthur through with his spear, but Arthur presses forward and pierces Mordred's enchanted armour with Excalibur, killing him.
At Arthur's behest, Perceval throws Excalibur in a pool of calm water, where it is caught by the scale-clad hand of the Lady of the Lake. When Perceval returns, he witnesses Arthur's body on a ship sailing away from the coast, attended by three formally posed ladies clad in white, sailing into the setting sun toward the Isle of Avalon.
Even though he was 35 years old, Nigel Terry plays King Arthur from his teenage years to his ending as an aged monarch.
Several members of the Boorman family also appeared in the picture. Igrayne (Arthur's mother), the Lady of the Lake, Mordred as a boy, and the infant Arthur were all played by Boorman's children. Because of the number of Boormans involved with the film, it is sometimes called "The Boorman Family Project."
John Boorman originally intended to film an adaptation of J.R.R. Tolkien 's The Lord of the Rings and much of the imagery and set designs seen in Excalibur were created with that intention. He was, however, unable to secure the rights to the book.
The original cut of the film was three hours long. Following a reduction in length, several shots were lost, among them a scene of Lancelot rescuing Guinevere from a forest bandit.
The film is primarily an adaptation of Malory's Morte d'Arthur (1485). In order to recast the Arthurian legends as an allegory of the cycle of birth, life and decay, the text was stripped of decorative or insignificant details, as well as of Malory's Christian piety. The resulting film is reminiscent of mythographic works such as Sir James Frazer's The Golden Bough and Jessie Weston's From Ritual to Romance; Arthur is presented as the "Wounded King" whose realm becomes a wasteland to be reborn thanks to the Grail, and may be compared to the Fisher (or Sinner) King, whose land also became a wasteland, and was also healed by Perceval. Notably, the Grail is not the Christian "Holy Grail"; rather, it may be inspired by magic cauldrons in Celtic pagan myths. "The film has to do with mythical truth, not historical truth," Boorman remarked to a journalist during filming.
In keeping with this approach, the film is intentionally ahistorical. For example, the opening titles state the setting to be the Dark Ages, even though the knights wear full plate armor, a technology of the 15th century. Knights, knighthood and the code of chivalry also did not exist during the period. Furthermore, Britain is never mentioned by name, only as "the land".
In addition to Malory, the writers incorporated elements from other Arthurian stories, sometimes altering them. For example, the sword between the sleeping lovers' bodies comes from the tales of Tristan and Iseult; the knight who returns Excalibur to the water is changed from Bedivere to Perceval; and Morgause and Morgan Le Fay are merged. The sword Excalibur and the Sword in the Stone are presented as the same thing; in some versions of the legends they are separate. In the Morte d'Arthur, Sir Galahad, the illegitimate son of Lancelot and Elaine of Carbonek, is actually the Knight who is worthy of the Holy Grail. Boorman follows the earlier version of the tale as told by Chretien de Troyes, making Percival the grail winner.
Merlin states the film's central theme, reflecting an ancient Celtic belief about kingship:
Later, he states the allegory of the Dragon:
Pallenberg and Boorman's screenplay touches on the heroic themes with directness. As Arthur declares:
As he prepares for his final battle, Arthur dreams of the future of his legacy: