In the ominous setting of 17th century Styria, twin sisters Maria and Frieda Gelhorn leave behind the luxurious canals of Venice and move to the menacing town of Karnstein to live with their strict Uncle Gustav Weil, the fervent leader of the witch-hunting Brotherhood. The sisters struggle against their uncle's rigid Puritanism, with Frieda yearning for freedom, drawn to the dark allure of the mysterious Count Karnstein, a figure whispered to dabble in Satanism and black magic. Unbeknownst to them, the Count's reality is far grimmer than mere gossip.
As the night progresses, Frieda's intrigue with the Count's ominous castle intensifies, prompting her to secretly venture out, leaving Maria to shield her absence. Inside the eerie castle, the Count preys upon Frieda, transforming her into a vampire and subjecting her to the horror of a chained victim—a gruesome gift. Back at home, Frieda's newfound hunger mandates her to intimidate Maria into continuing her deceit, fearing the dreadful possibility of losing control and harming her beloved sister—a secret she keeps closely guarded.
Maria, meanwhile, finds herself captivated by the charming young teacher Anton, who harbors an intellectual curiosity for superstition. As Anton becomes enchanted with Frieda's enigmatic nature, he soon encounters the ghastly truth of vampiric existence when tragedy strikes, putting his sister in peril. As night falls further, Frieda, now a creature of the night, assaults a member of the Brotherhood, leading to her imprisonment at the hands of Uncle Weil. While the Brotherhood deliberates her fate, the Count and his disturbing followers devise a daring plan, trading Frieda for Maria, who languishes in captivity. Unaware of this switch, Anton confronts what he believes to be the charming Maria, only to be shaken by her inability to reflect in a mirror and her revulsion to a crucifix. His infatuation turns to horror as he realizes the truth, thrusting him into a frantic race to save his true sister from a fiery death, all while the real Frieda masquerades as Maria.
As the indomitable Weil takes heed of Anton's advice on eradicating the undead, he, alongside the valiant Anton, rallies a band of determined Brotherhood warriors and brave villagers to embark on a noble quest to overthrow the malevolent Count Karnstein at his foreboding castle. The once-imposing Castle Karnstein now stands as the epicenter of their righteous battle against the encroaching darkness. As Frieda, now the Countess, seeks to escape with her sinister paramour, Count Karnstein, they confront the relentless resolve of Weil. Undeterred by the dangerous locale and the vile forces that dwell within, Weil captures Frieda, abruptly halting her desperate escape. In a dramatic showdown, the Count seizes the defenseless Maria, only to find his plans thwarted by Weil's fearless emergence, wielding a formidable axe. With determination, Weil faces off against the Count, ultimately sacrificing himself to the vengeful vampire lord. This act of valor empowers Anton to strike the decisive blow, plunging his spear into the heart of darkness—Count Karnstein—thus annihilating the evil that has plagued their world.
As the settlement of dust reveals the aftermath of this harrowing conflict, Maria finds solace with Anton, while the once-majestic Castle Karnstein, now ravaged by time and turmoil, finally relinquishes its grip on malevolence.
