One of the most financially successful films of 1981, Excalibur is a retelling of the classic myth of King Arthur. The film is based on the Thomas Malory version of the legend, which is titled Le Morte d'Arthur.

The film begins with England engaged in civil war, a generation before the birth of Arthur. The wizard Merlin (Nicol Williamson) receives the legendary sword Excalibur from The Lady of the Lake, which he in turn gives to Uthur Pendragon (Gabriel Byrne). Uthur uses the sword to subdue the Duke of Cornwall's forces. Merlin then helps Uthur disguise himself, infiltrate Cornwall's castle, and sleep with his wife Igrayne (Katrine Boorman). Arthur is conceived as a result. Before Uthur is later killed by Cornwall's remaining forces, he locks Excalibur in a stone, declaring that whoever pulls the sword from the stone will be the future king of England. Arthur (Nigel Terry) grows to claim the sword and his kingship, and through the tutelage of Merlin, marries Guenevere (Cherie Lunghi) and establishes the Knights of the Round Table. All seems to be unfolding perfectly for Arthur, until his half-sister Morgana has a child by him, which sets future events into motion that could destroy Camelot.