Dr. Abner Perry (Peter Cushing) is an English scientist who has invented a massive drill that can dig deep into the earth. Joined by his adventure-seeking American backer, David Innes (Doug McClure), Perry accidentally discovers an underground civilization of primitive humans who live in fear of telepathic flying monsters. David becomes intrigued by the beautiful native Dia (Caroline Munro), and must rescue her when she is chosen as a human sacrifice.