The narrative begins with an intellectual framing device examining the peculiar habits of Lord Byron in Switzerland and the circumstances that inspired several ghost stories among his circle. From this atmospheric preamble, the focus shifts to London, where the enigmatic and coldly alluring Lord Ruthven appears in fashionable drawing-rooms. Despite his unsettling demeanor and dead gray eyes, he commands intense curiosity and effortlessly manipulates those around him, lavishing unpredictable charity that invariably ruins the recipients.
Aubrey, an idealistic and wealthy young orphan with a head full of romantic notions, becomes fascinated by Ruthven's singular personality. When Ruthven prepares to travel, Aubrey's guardians suggest a continental tour, leading the two men to journey together across Europe. Throughout their travels, Aubrey observes Ruthven's disturbing benevolence toward the vicious and his predatory enjoyment of high-stakes gambling, which routinely drives innocent men to ruin and despair. Troubled by these ominous patterns, Aubrey eventually discovers the darker truth behind his companion's reputation regarding the systematic degradation of virtuous women.
Determined to break ties, Aubrey abandons Ruthven in Rome and travels onward to Greece, seeking solace among ancient ruins. There he meets Ianthe, a pure and spirited Greek girl whose naive charm captivates him, contrasting sharply with the affected manners of English society. Ianthe shares local folklore with him, particularly chilling tales of living vampyres who prey upon those they once loved to sustain their unnatural existence. Despite his lingering skepticism, Aubrey notices terrifying parallels between these regional legends and Lord Ruthven's uncanny nature.
Tragedy strikes when Aubrey is caught in a violent storm during a solitary excursion and takes shelter in a remote hovel. Inside, he hears horrifying shrieks and discovers Ianthe being brutally attacked by a superhuman assailant before the creature escapes into the night. Ianthe is brought back lifeless, bearing the fatal mark of a vampyre's bite, which shatters Aubrey's health and plunges him into a prolonged feverish delirium. During his recovery, Ruthven reappears and tends to him with an unnerving return of attentive care.
Recovering his physical strength but broken in spirit, Aubrey continues his travels with Ruthven until they are ambushed by robbers in a mountain pass. Ruthven receives a mortal wound and extracts a solemn, terrifying oath from Aubrey, binding him by everything he holds sacred to conceal his death and crimes for a full year and a day. Shortly after exacting this vow, Ruthven dies, though his corpse mysteriously vanishes from the mountaintop where it was exposed.
Returning to England burdened by trauma and his binding oath, Aubrey attempts to shield his young sister from the world. However, Ruthven reappears in London society completely unharmed, targeting Aubrey's sister with the same calculated seduction he used on past victims. Bound by his vow, Aubrey is rendered powerless to warn anyone openly, watching in agony as his sister falls under the monster's spell and accepts his hand in marriage. The story culminates in a final, devastating tragedy as the binding promise expires too late to save the bride from the vampyre's deadly thirst.
