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.

Lancelot lies wounded in a hermit's hut. Guinevere has innocently caused the death of a Knight of the Round Table by giving him a poisoned peach from a bowl, placed there by Morgan and intended to be eaten by Guinevere.