Southern France, Provence. Iolanta, blind from birth, is the only daughter of King René. She is the fiancée of Robert, Duke of Burgundy. Iolanta’s father hopes to have his daughter cured by the time of the wedding and it has been announced that Iolanta is being educated in a Spanish nunnery.
René doesn’t want his daughter to know about her ailment. He’s forbidden to make any mention of the light, colours, sights: she shouldn’t know about her blindness. Everybody is kind and tender to Iolanta but she’s been inexplicably sad lately.
Ibn-Hakia, a famous Moorish physician, has been invited to the castle. He states that Iolanta can be cured but only in case she is informed of her blindness and has a desire to see. The king refuses the treatment, fearing for Iolanta’s happiness.
Two friends, knight Vaudémont and count Robert have lost their way and suddenly come to the King René’s castle. Robert announces to Vaudémont his love to beautiful Mathilde, countess of Lorraine. But since childhood, he’s been engaged to Iolanta, King René’s daughter.
Vaudémont encounters Iolanta who is asleep. Stunned by her beauty he falls in love.
Iolanta hears the voices and wakes up: they rarely have guests in the castle. The young men explain that they have come here by chance. Robert leaves to find his people, Iolanta and Vaudémont stay alone. He confesses his sudden love for her.
He discovers Iolanta’s blindness and her ignorance of the fact and explains light and color to her. The couple is discovered by the king, the physician and Iolanta’s friends. They find out that the secret has been revealed to the girl. The king is desperate, Ibn-Hakia, however, is sure that it’ll save Iolanta.
Iolanta can’t wish to see, as she doesn’t know what it is. The king threatens to kill Vaudémont if the physician’s treatment fails. Iolanta is ready to do anything to save the knight. Vaudémont asks the King to marry his daughter who refuses as his daughter has been engaged since her childhood.
Robert returns to find out that Vaudémont’s beloved girl is his fiancée. Vaudémont begs Robert to admit his love to Mathilde. The king cancels the wedding contract. Ibn-Hakia takes off a bandage from Iolanta’s eyes: she can see now. She’s frightened, she can’t recognize anybody. King René comforts Iolanta and offers her a reliable partner: Vaudémont who pledges fidelity to Iolanta. Everybody glorifies God.