Dr. Sarah Chapel returns to the small town of Brighton Mills when her father Dr. Robert Chapel unexpectedly dies. His clients and friends miss him, and Sarah discloses that all of them have perfect health; they hear voices when they are sleeping; and they are addicted to the stimulant Tributol. Her further investigation discloses a dark secret about the haunting voices.

American tourists David and Jack are savaged by an unidentified vicious animal whilst hiking on the Yorkshire Moors. Retiring to the home of a beautiful nurse to recuperate, David soon experiences disturbing changes to his mind and body.