John Boorman retells the legend of King Arthur, based solely on the 15th century Arthurian romance Le Morte d'Arthur by Thomas Malory.

The film is named after the legendary sword of King Arthur that features prominently in Arthurian literature. Uthur Pendragon is given the mystical sword Excalibur by the wizard Merlin. At his death Uthur buries the sword into a stone, and the next man that can pull it out will be King of England. Years later Arthur, Uthur's bastard son draws Excalibur and becomes king. Guided by Merlin, Arthur marries Guenivere and gathers the Knights of the Round Table. Arthur's evil half-sister Morgana sires a son with him, who may prove his downfall.