In a quiet German village along the Rhine, a boarding school becomes the focal point of a chilling mystery as a string of grisly murders targets its female students every full moon. The locals point to Lorelei Helga Liné, a legendary water spirit said to dwell in a grotto beneath the Rhine and to transform from a beautiful woman into a reptilian, bloodthirsty monster under the moonlight. The town’s fear grows with each passing cycle, and the air fills with whispered legends and uneasy tension.
To protect the school, an experienced hunter named Sigurd [Tony Kendall] is hired by the school’s teacher, Elke Ackerman [Silvia Tortosa]. He imposes a strict 9 o’clock curfew, orders all doors and windows to be secured, and forbids anyone from leaving the grounds during that window. The atmosphere turns claustrophobic as the moon climbs, and the students huddle within the locked walls, wondering who will be next.
One night, while patrolling the grounds, Sigurd encounters a cloaked, mysterious woman who slips away when she notices him. Later, during a swim in a nearby lake, he spies her again but cannot catch her. In his search, he crosses paths with Professor Von Lander [Ángel Menéndez], a researcher who has been studying the creature for some time. In his lab, Von Lander reveals a stark truth: Lorelei can transform into a monster by the light of the full moon, and he has even crafted an irradiated dagger that could revert her to human form. The tension between belief and disbelief tightens the plot as the danger grows.
A separate thread shows the creature’s violence: a small hunting party is slaughtered, and the cloaked woman is revealed to be Lorelei. Yet Sigurd, drawn by an inexplicable attachment, refuses to acknowledge her guilt and is slow to accept the truth about her. His emotional entanglement blinds him to Von Lander’s potential complicity, setting the stage for a dramatic confrontation.
Lorelei, aided by her human servant Alberic [Luis Barboo], murders Von Lander and destroys his research to prevent anyone from stopping her. With the scientist erased, Sigurd’s suspicion sharpens into a stubborn resolve as he follows a trail that leads to an underwater cave. There, an opening reveals a hidden temple inhabited by Lorelei, Alberic, and a horde of feral women who serve the dark queen of the grotto.
Sigurd is captured after being discovered by Lorelei’s followers, and the stakes escalate as Lorelei heads to the boarding school to claim Elke, driven by the revelation of Sigurd’s feelings. A feral woman—a tragic ally—helps Sigurd escape, but the temple and its feral populace perish as the dynamite Sigurd had set elsewhere explodes, taking the threat with it.
With the danger momentarily quelled, Sigurd arrives at the boarding house carrying the irradiated dagger. In a final, decisive confrontation, he uses the weapon to slay Lorelei just as she moves to claim Elke, bringing an end to the bloody cycle and restoring a fragile peace to the school and the village.
