Aubrey, a young Englishman, meets Lord Ruthven, who has recently entered London society. The two travel together to Rome, where Ruthven seduces the daughter of a mutual acquaintance. Aubrey leaves Ruthven and travels to Greece, where he meets Ianthe, the daughter of an innkeeper. Ianthe tells Aubrey the legend of vampires. Shortly afterwards, Ianthe is murdered by Ruthven, who had arrived in Greece as well. Aubrey doesn't connect the murder to Ruthven, however.
Ruthven rejoins Aubrey in his travels, but the two are ambushed by bandits and Ruthven is mortally wounded. Before apparently dying, Ruthven forces Aubrey to swear on an oath not to mention him for a year and a day. Aubrey agrees and returns to London.
Aubrey is amazed when Ruthven returns, alive and well. Ruthven reminds Aubrey of his oath to keep his death a secret, and begins to seduce Aubrey's sister. Aubrey, powerless to save his sister, undergoes a nervous breakdown. On the day the oath ends, Ruthven and Aubrey's sister are engaged to marry. Before he dies, Aubrey writes a letter to his sister revealing Ruthven's history, but the letter doesn't reach her in time. Ruthven marries Aubrey's sister, and on their wedding night, kills her and drains her off her blood before escaping.
At a Witches' Sabbath, the Vampire Master tells Lord Ruthven that if he cannot sacrifice three virgin brides within the next 24 hours, he will die. If he can, he will be granted another year of life. The clock strikes one, and Ruthven's first victim, Janthe, arrives for a clandestine meeting, although she is due to marry another on the following day. Berkley, having discovered that she is missing, is searching for her with his men, and Ruthven hides with her in a cave. Her screams alert the search-party, and the body and the Vampire are discovered. Berkley stabs Ruthven and leaves him to die, but he is discovered by Aubry, whose life had been saved by Ruthven in the past. Ruthven pleads with Aubry to drag him into the moonlight so that he can revive, and Aubry, while doing so, realises that Ruthven is a vampire. He has to swear not to reveal this secret for twenty-four hours, or he will become a vampire, too.
The 18-year-old Malwina and Aubry, with whom she is in love, are told by Davenaut that she must marry the Earl of Marsden. Aubry recognises the Earl as Lord Ruthven, but is told that he is Ruthven's brother, who has been abroad for some time. Aubry, however, recognises a wound that proves that the Earl really is Ruthven, and is about to denounce him when Ruthven reminds him of his oath and the consequences that will follow if he breaks it. The preparations for Malwina's marriage to "Marsden" begin.
Emmy awaits her husband-to-be, George. News of Janthe's gruesome death emerges, and Emmy recounts the legend of the Vampire. Ruthven appears and impresses the villagers with his largesse. He flirts with Emmy until, interrupted by George, he departs - though by then he has extracted a promise from Emmy that she will dance with him later. Aubry tries to persuade Ruthven to give up his claim to Malwina, but is again reminded of the fate that awaits if he breaks his oath. Ruthven, in a soliloquy, rails against the torments that a Vampire must face.
Aubry is torn by his choice between breaking his oath and saving Malwina, or keeping quiet and losing her to the Vampire. George asks Aubry to use his influence to stop "Marsden" from seducing Emmy. Aubry warns George that he must keep watch over Emmy - but already she is being led into the forest by Ruthven. Blunt, Gadshill, Scrop and Green sing of the pleasures of drink. Blunt's wife Suse upbraids the men, to the delight of the onlookers, but a dishevelled George arrives, recounting how he followed Emmy and "Marsden", only to find him standing over her dead body. He had shot the Earl immediately, leaving him to die in the moonlight. The villagers express their sympathy and sorrow.
Malwina is to be married to "Marsden" before midnight. Aubry warns her that she is in danger, and she puts her trust in God. The wedding-guests arrive, followed by Ruthven, who apologises for his lateness. Malwina and Aubry make one last appeal to Davenaut, who throws Aubry out and orders the wedding to proceed. A thunderstorm approaches, and Aubry returns, having decided to reveal Ruthven's secret at no matter what cost to himself. Suddenly, the clock strikes one, and Aubry, released from his oath, reveals that "Marsden" is Lord Ruthven, the Vampire. Ruthven, having failed in his task, is struck by lightning and descends into Hell. Now Davenaut asks Malwina to forgive him and consents to her marriage to Aubry, to general rejoicing.