In the opera, Merlin's power in the supernatural comes from a magic harp, with which Merlin controls a demon who reluctantly does Merlin's bidding. While the demon bemoans his fate as Merlin's slave, the fairy Morgana offers to the demon this advice: "Only through the love of a woman can Merlin's powers be extinguished." After using his magic to win a victory over the Saxons for Arthur, Merlin identifies to Arthur a traitor, Bedwyr.
A nice, young girl, Viviane, appears in Arthur's court. She and Merlin are immediately attracted to one another. And when Viviane touches Merlin's hand, he unknowingly loses all of his magical powers. Modred, King Arthur's nephew, plots with Bedwyr against Arthur. Lancelot warns Arthur about Modred. But when Merlin is summoned to ascertain Modred's intentions, Merlin can find no guilt in him because Merlin, unknowingly, no longer has the power.Then, the demon steals a magic veil from Merlin's temple and gives it to Viviane. Merlin is very upset when he finds Viviane with his magic veil. And he explains to her that those whose brow is touched by the veil are condemned to stay where they are forever. Though Merlin and Viviane express their love for each other, Viviane mistrusts Merlin's vow and puts the magic veil on Merlin's brow, causing Merlin to instantly be bound in chains and the demon to be set free. Morgana then regrets her role in helping the demon and she tells Viviane that: "Love stronger than death will conquer the power of evil."
Lancelot, not understanding Merlin's predicament and not being able to see the chains, asks Merlin to help Arthur in his battle against Modred. Merlin is unable to free himself from the chains. Suddenly, the demon appears and frees Merlin, who then joins the battle and is mortally wounded. Merlin tells Viviane, as he dies, that he had to agree to give up his life to have the chains removed. Viviane realizes that through death she will be united with Merlin forever, and she stabs herself.