A pathologist, Dr. Warren Chapin (Price), discovers that the tingling of the spine in states of extreme fear is due to the growth of a creature that every human being seems to have, called a "tingler", a parasite attached to the human spine. It curls up, feeds and grows stronger when its host is afraid, effectively crushing the person's spine if curled up long enough. The host can weaken the creature and stop its curling by screaming.