Merlin has given Arthur the magic sword Excalibur to help him defeat the nobles who oppose his rule. Unknown to Arthur, his half-sister Morgan, a sorceress masquerading as a nun, has sworn to kill him in revenge for her father's death.

Arthur has made himself ruler of all Britain, created the Fellowship of the Round Table and made Guinevere his queen. Morgan, determined to destroy him, has stolen his scabbard, which affords magical protection against wounds, and thrown it into a lake.

Morgan, feigning sisterly love, has intensified her campaign against Arthur. She has used magic to deceive Lancelot into thinking he has made love to Guinevere; and, disguised as a nun, has told him he must never forget his guilt.