For centuries, a legendary monster has terrorized residents of a Bavarian castle, called Castle Finster, in Germany. According to the legend, the monster will vanish for years at a time until, without warning, it reappears to claim its next victim. So far every victim it has claimed has been a young woman, reputed to be wearing a peculiar jeweled necklace at the time of her disappearance. In the present day, young American detective Nancy Drew has been called in by the castle's owner to discover the truth behind recent monster sightings, before the monster can strike again. As Nancy delves deeper into the case, she begins to fear that someone in the castle plans for her to be the monster's next victim.