En route to taking up a position in Transylvania, Marianne Danielle, Yvonne Monlaur, a French schoolteacher, is abandoned at a village inn by her coach driver. Ignoring the locals’ warnings, she accepts the invitation of Baroness Meinster, Martita Hunt, to spend the night at her castle. Inside, she is shocked to encounter Baron Meinster, David Peel, a supposed madman kept chained to a wall by his mother. He pleads that his mother has wrongfully usurped his lands and begs Marianne for help. In a bold move, Marianne steals the key from the Baroness’ bedroom and frees him, unwittingly setting a series of dire consequences in motion.
The freed baron soon asserts his strength, killing his mother and drinking her blood, a brutal act that shatters the castle’s veneer of safety. Greta, Freda Jackson, the castle’s loyal servant, blames the late Baroness for having allowed the baron to be turned into a vampire by Dracula, and she remains steadfastly loyal to Meinster. The chilling events leave Marianne shaken, and Greta’s stubborn loyalty to the Baronet hints at the darkness looming over the village. At dawn, Marianne is exhausted and found by Dr. J. Van Helsing, Peter Cushing, who guides her to the school where she will teach, while the investigator within him senses that evil has only begun to stir.
Van Helsing returns to the village inn and discovers a funeral in progress: a young girl has been found dead in the woods, slashed with wounds on her throat. Father Stepnik, Fred Johnson, asks the doctor to help, and the two try to persuade the girl’s grieving father not to bury her too soon. He does not listen, and the girl eventually rises as a vampire, prompting Stepnik and Van Helsing to race to the cemetery that night. Greta, still human for the moment, encourages the newly risen girl to rise from the grave, hampering the men’s efforts and allowing the creature to slip away. The events force Van Helsing to confront the castle again, where he finds Baroness Meinster now risen as a vampire and the Baron himself still at large. After a brief clash, the Baron escapes, leaving his mother to her fate.
Van Helsing, moved by the baron’s twisted cruelty, confronts the dead Baroness in the morning and shows mercy by putting her to rest with a wooden stake. Meanwhile, the Baron Meinster visits Marianne at the school and asks her to marry him. She accepts, much to the envy of her roommate Gina, Andrée Melly. The Baron later enters Gina’s room and drains her blood, a stark turn that deepens the peril facing Marianne and the village.
The next day, Van Helsing examines Gina’s body and orders that she be kept under strict watch in a horse stable. Marianne keeps vigil that night, aided by Severin, Harold Scott, the steadfast stable keeper. A vampirised Gina rises from her coffin and advances on Marianne, but she is driven back as the details of Gina’s warning push Marianne to reveal the Baron’s hideout at the old mill. Van Helsing races to the mill and confronts the pair of the Baron's brides as well as the Baron's own attempts to subdue him, warded off momentarily by his cross. Greta, still loyal but doomed, is dragged down in the struggle, and the Baron himself bites Van Helsing, spreading vampirism through him before the light of day.
Yet Van Helsing’s resolve remains unshaken. He returns to the brazier, heats a metal tool, and uses holy water to seal the wound, slowly purging the vampiric infection. The Baron then abducts Marianne and drags her to the mill, attempting to hypnotize her into submission. The holy water scorches his face, a sign that his strength can be overcome. In a desperate turn, he overturns the brazier of hot coals and dashes outside as his brides flee the burning building. Van Helsing directs the sails to cast the shadow of a cross over Baron Meinster, and the symbol proves lethal to him, finally ending his reign of terror.
In the aftermath, Van Helsing consoles Marianne as the mill burns around them, the village saved from a nightmare born of a broken family’s hunger for power and blood. The film closes on a note of cautious relief, with Marianne’s future once again in human hands and Van Helsing’s vigilance intact against the lingering darkness of Transylvania.
