In a secluded village in Styria, a mysterious woman named Marcilla befriends General Spielsdorf’s niece, Laura, at a lavish ball. Marcilla appears overnight at the family’s home after her mother leaves abruptly, claiming she must go away. Initially charming and seemingly benign, Marcilla's presence quickly coincides with Laura's decline in health marked by dreams of a giant cat attacking her. Despite various attempts to help Laura, her condition worsens, culminating in her untimely death. Marcilla disappears, leaving the family baffled and grieving.

Soon after, a similar pattern unfolds when a young woman named Carmilla arrives at the Morton family's estate after a supposed carriage accident. She befriends Emma Morton, who begins suffering the same symptoms that afflicted Laura. Emma experiences terrifying nightmares and a steady physical decline. The Mortons, alarmed by the similarities to recent events in the area, team up with General Spielsdorf to uncover the truth. They consult with medical and clerical authorities, who reveal ancient legends and the dark secret behind Marcilla's true identity. Determined to save Emma and put an end to the terror, the group ventures into perilous territory to confront the malevolent force preying on their loved ones.