Deep in a famous New Orleans cemetery, the Vampire Master tells the vampire Ruthven that he will die if he cannot offer three brides as sacrifice within the next 24 hours. If he can, he will be granted another year of life. The clock strikes midnight, and, though she is to marry another man the following day, Ruthven’s first young victim, Janthe, arrives for a clandestine meeting. Her father, Berkley, has discovered that she is missing and is searching for her with his men. She and Ruthven hide and her screams soon alert the searchers who discover the vampire standing over her lifeless body. In a rage, Berkley shoots Ruthven and leaves him to die. The dying vampire is discovered by Edgar Aubry, a young man whose life had been saved by Ruthven in the past and who has sworn a sacred oath to repay this debt. When Ruthven pleads with Aubry to drag him into the moonlight so that he can revive, Aubrey comes to the horrible realization that Ruthven is a vampire. He is forced to swear not to reveal this secret or he will be struck down by God for violating his oath.
The next morning, Malwina Davenaut and Aubry resolve to ask her father if they can marry. Davenaut, a successful social-climbing businessman who desperately wants his daughter to marry into high society, tells her that he has decided that instead she will marry his important business partner Ruthven Marsden. When introduced, Aubrey is certain that he recognizes Marsden, who says that he has been abroad for some time and that Aubrey has confused him with his brother. However, Aubry recognizes signs that prove that this man is really the vampire. Aubrey quietly threatens to reveal him, but Ruthven reminds him of his oath and the consequences for breaking it. Friends have now gathered at the Davenaut home to celebrate Malwina’s birthday, and her father grandly announces that her marriage to Ruthven Marsden will take place that very evening.
With her wedding reception joyously underway in the French Quarter, Emmy, the daughter of Marsden’s business manager, awaits the arrival of her new groom George. At the party, news of Janthe’s gruesome death is mentioned, so Emmy regales her friends with the colorful legend of the vampire. Ruthven suddenly appears and generously insists on picking up the bill for the occasion, impressing the group with his largesse. Emmy is mesmerized by Ruthven’s wealth and social class and very flattered by all the attention he directs toward her. Ruthven flirts with Emmy until, interrupted by George’s arrival, he departs—but not before extracting a promise from Emmy that she will dance with him later.
Aubry has followed Ruthven to persuade him to give up his claim to marry Malwina, but is reminded of his fate if he breaks his oath. In a powerful soliloquy, Ruthven rails against the torments that a vampire faces, both on earth and in Hell. The need for blood means that you can live only by destroying those around you—including those you love. Even in Hell, the spirits of the damned are terrified of vampires. It’s obviously sad to have so few friends. Ruthven leaves and Aubry is torn by the difficult choice to break his oath to save Malwina or remain silent and lose her to the vampire. George asks Aubry’s help to protect Emmy from the alluring Marsden, but already she is being led astray by the seductive vampire. Four revelers at the party sing of the pleasures of drink. Emmy’s mother, Suse, sarcastically upbraids the men to the delight of the onlookers, but the comic antics are interrupted by the sound of gunshots. Devastated, George arrives to recount how he followed Emmy and Marsden and found him standing over her dead body. He shot Ruthven, but he escaped into the moonlight. The guests and friends are shocked into sympathy and sorrow.
Preparations are complete for Malwina to marry Ruthven Marsden before midnight in the grandeur of St. Louis cathedral. Aubry warns her that she is in danger, but she puts her trust in God. The colorful wedding procession in Jackson Square begins after the many guests arrive, followed by Ruthven, who apologizes for his lateness. (Remember, he had been detained with Emmy.) Malwina and Aubry make one last impassioned appeal to Davenaut, who, thinking that Aubrey has lost his sanity over the marriage decision, has him thrown out of the occasion. In anger at his daughter’s continuing resistance, he issues to Malwina a withering father’s curse and commands that the wedding proceed. In a final desperate attempt to stop the wedding, Aubry forces his way back in, willing to reveal Ruthven’s secret at whatever cost to himself. Just as the clock strikes midnight, Aubry reveals that Marsden is actually that “horror of nature,” a vampire! With this revelation, Ruthven is struck by a bolt of divine lightning and descends into Hell. Before the totally amazed wedding guests, Davenaut asks his daughter’s forgiveness and eagerly consents to her marriage to Aubry, a decision met with great rejoicing and a closing chorus thanking the Almighty for protecting man from evil.