A mysterious monster is turning people to stone in a German village in 1910. When his girlfriend is killed, Bruno (Jeremy Longhurst) becomes the prime suspect. His ensuing suicide seems to confirm his guilt, but professor Carl Maister (Christopher Lee) isn't so sure. He thinks one of the villagers is possessed by the spirit of Megaera, sister to Medusa. Among the possible culprits are Dr. Namaroff (Peter Cushing), gorgeous nurse Carla (Barbara Shelley) and a mental patient.