Queen Guinevere is torn between love for her husband and Sir Lancelot.

In 6th century England, the end of the Roman occupation has left the country in turmoil. At the site of a recent battle between warring overlords, Arthur Pendragon and his half-sister, Morgan LeFay, meet as arranged by the sorcerer Merlin to discuss how to end the bloodshed. Morgan maintains that as she is the only legitimate offspring of the late king, the throne belongs to her, but Merlin puts the adversaries to a test to determine England's rightful ruler. Merlin leads them to Excalibur, a sword embedded in an anvil, and says that according to legend, whoever can remove the sword shall be England's true sovereign. Morgan's knight champion Modred tries in vain to extract the sword, but Arthur removes it easily. Modred accuses Merlin of witchcraft, and a hearing is arranged with the Council of Kings at the circle of stone. After advising Arthur that he must prove himself worthy of the throne by his deeds, Merlin instructs him to return the sword to the stone. Meanwhile, the French knight Lancelot and his men ride toward the circle of stone, hoping to offer their services to Arthur. On the road, Lancelot encounters a young woman named Elaine, who quickly falls in love with the handsome knight. They are waylaid by Modred's men, and Lancelot bravely does battle with all of them. Arthur arrives and join in the battle, and the chagrined Lancelot, unaware of Arthur's identity, challenges him to fight. After a long, exhausting battle, Lancelot finally asks his opponent's name, and when he learns it is Arthur, breaks his sword against a tree and kneels before him. They are joined by Elaine's brother Percival, who asks to be Arthur's knight errant. Later, at the circle of stone, Arthur and Modred debate before the Council of Kings. When the crowd turns against Arthur and Lancelot, they are forced to flee, vowing to take the kingdom on the battlefield. Arthur and his men lie low throughout the cruel winter, then launch their attack against Modred's men in the spring. Despite being greatly outnumbered, Arthur's men win, and Arthur is crowned King of England. In the interest of peace, Arthur immediately pardons all his former enemies, but when Lancelot objects to Modred's pardon, the two men angrily part ways. On the road, Lancelot discovers that the lovely Guinevere has been kidnapped and rescues her, unaware that she is Arthur's fiancée. Arthur and Guinevere are wed, and the king's joy is complete when Lancelot arrives at Camelot and pledges his allegiance anew. Arthur swears in his select group of knights at the Round Table, and England enjoys a period of peace and prosperity. One day, Percival brings Elaine to court and asks Guinevere to make her a lady-in-waiting. Meanwhile, Morgan and Modred continue to harbor ill feelings against Arthur, and note with interest the growing warmth between Lancelot and Guinevere. Merlin privately warns Guinevere that Modred will attempt to sow suspicion about her relationship with the knight, and says that Lancelot should marry. Guinevere tells Lancelot she knows of his secret love for her and urges him to marry Elaine. Lancelot proposes to Elaine and asks Arthur to let him join the fight at the Scottish border. One night, Lancelot and Elaine are visited by Percival, who relates how a heavenly vision instructed him to go on a quest for the Holy Grail, the cup from which Christ drank at the Last Supper. Modred calls a meeting of Arthur's enemies in Scotland and urges them to make peace so that Lancelot will be forced to return to Camelot, where he will eventually be exposed as Guinevere's lover. Word of peace reaches Arthur at Camelot at the same time that Lancelot's infant son Galahad, whose mother died in childbirth, is brought to court with instructions that he be sent to Lancelot's father. Sensing a plot, Merlin argues against bringing Lancelot back to Camelot, but Morgan poisons him, and the knight returns amid great fanfare. Late one night, jealous after seeing Lancelot kiss another woman, Guinevere goes to his rooms, unaware that she is being spied on by Morgan and Modred. Lancelot angrily denounces Guinevere's folly in coming to him, and Modred's men soon arrive to arrest them for high treason. Lancelot fights them off and flees with Guinevere. Lancelot and Guinevere are tried in absentia at the Round Table and declared guilty. Lancelot suddenly walks in and surrenders, and when he confesses his chaste love for Guinevere, Arthur revokes their death sentence. Over Modred's protests, Arthur orders that Guinevere be confined in a convent and banishes Lancelot from England. Outraged at this show of mercy, Modred succeeds in turning the other knights against Arthur, and civil war returns to the land. Arthur meets with Modred and agrees to his terms for ending the war, which include disbanding the Round Table. When one of Arthur's men draws his sword to kill a snake, however, the battle cry is sounded. Arthur is mortally wounded, and Lancelot returns from exile to be at his side. With his dying breath, Arthur commands Lancelot to destroy Modred and give Guinevere his love and forgiveness. Pausing only to hurl Excalibur into a lake, Lancelot calls on Guinevere at the convent and conveys Arthur's message, then kills Modred after a fierce battle. Lancelot then meets Percival at the Round Table and weeps, blaming himself for the noble fellowship's demise. After Percival receives another holy vision of the Grail, he hears God's voice telling him that Lancelot's son Galahad will be a worthy knight, and that Lancelot is forgiven and will now know peace.