A brilliant geologist sends a team to search the depths of the earth in an effort to learn the origin of some strange signals they've been picking up on their equipment. They discover a rock formation with an interior life that can communicate telepathically. They bring the mass to the surface and determine that the entity is "pure crystallized intelligence," and they believe many of the secrets of existence can be answered by studying the stone. However, the only message the mysterious rock is forthcoming with is what kind of food it needs, namely a particular chemical that can only be produced by the bodies of people in the throes of terror. To this end, Dr. Mantel (Boris Karloff) devises something called the Fear Chamber, where kidnapped prostitutes are frightened out of their wits with spiders, snakes, and faked Satanic rituals in order to obtain this precious chemical. Though the scientist and his assistants take pains not to physically harm their subjects, the weird living rock begins growing tentacles and soon learns how to capture its prey and extract the fluids itself, leading to the death of a lady burglar. The scientists realize what a deadly force they've unleashed on the surface world, though efforts to destroy the creature are complicated by its ability to control human minds and generate its own electricity.