Mary Henry (Hilligoss) is riding in a car with two other young women when some men challenge them to a drag race. As they speed across a bridge, the women’s car plunges over the side into the river. The police spend three hours dragging the murky, fast-running water without success. Mary miraculously surfaces, but she cannot remember how she survived.
In the Florida Everglades, a pair of larger-than-human, intelligent Leech are living in an underwater cave. They begin dragging local people down to their cave where they hold them prisoner and slowly drain them of blood.
When the villagers of Kleinschloss start dying of blood loss, the town fathers suspect a resurgence of vampirism, however police inspector Karl Breettschneider remains skeptical. Scientist Dr. Otto von Niemann, who cares for the victims, visits a patient who was attacked by a bat, Martha Mueller. Martha is visited by a mentally challenged man
Eccentric millionaire Frederick Loren (Vincent Price) invites five people to a “party” he is throwing for his fourth wife, Annabelle (Carol Ohmart), in an allegedly haunted house he has rented, promising to give them each $10,000 with the stipulation that they must stay the entire night in the house after the doors are locked at midnight.