A modest villa in the vicinity of a town. Melusine leads an innocent, dreamy life with her husband, Oleander. She spends days and sometimes whole nights in the lovely park which surrounds their house. Her mother, Madame Lapérouse, criticizes her for this. But the park is sold to Count Lusignan, and when a surveyor comes to prepare the building of a castle in it he is driven wild by Melusine’s promise to be his if he will only leave the park untouched. He falls to his death from one of the surrounding walls.

Scene I: In the park. Melusine seeks counsel from Pythia, the ruler of the spirits of nature disguised as a fortune teller, as to how she can avert the destruction of the park. Pythia grants her the magic fish-tail, “…which will protect and sanctify you. It will make you irresistible to all men, but you will remain pure despite their excesses.” Pythia warns her, however, that only complete chastity will make the magic work to help her achieve her aims. A mason is driven mad by Melusine’s newly found powers.

Scene II: Near a lake in the park. The building of the castle is making no progress. Strange things are happening. The workers strike. The architect is in despair. Cries for help are heard from the lake. The architect rescues Melusine from drowning and falls in love with her.

Scene I: In front of the castle. The completion of the castle is being celebrated. Pythia and her accomplice Ogre appear. He lets himself be taken on as a lackey to be able to protect Melusine during the celebrations.

Scene II: A hall in the castle. Count Lusignan, who had the castle built, meets Melusine for the first time at the reception. They fall in love. The architect storms in and warns the Count of Melusine’s magic powers, but Ogre throws him out. Pythia witnesses the complete fascination of the Count and Melusine for each other.

Scene I: A road in the park. Melusine meets the Count in the park.

Scene II: Bedroom in the castle. Melusine has lost her innocence and regrets it already. When the Count leaves for a moment, Ogre enters and curses Melusine for her betrayal. Pythia has set fire to the park and the castle so as to destroy them all. Melusine dashes into the flames to try to save her lover.

Scene III: The villa as in Act I. Oleander believes that the Count is putting on a fireworks display The Count’s secretary enters and, telling of the fire, says that the Count and his mistress have perished in the flames. Oleander realizes that she was none other than Melusine.