Biologists Dr. Mary Robinson (Janice Logan) and Dr. Bullfinch (Charles Halton) are summoned by Dr. Alexander Thorkel (Albert Dekker) to his remote laboratory in the Peruvian jungle. Thorkel’s work is shrouded in mystery so they jump at the chance to go.
They embark on their journey accompanied by mineralogist Dr. Bill Stockton (Thomas Coley), a friend of Mary’s and a last minute substitute for another scientist, and Steve Baker (Victor Kilian), who wants to make sure his hired mules are well cared for (and suspects Thorkel may have discovered a rich mine).
Upon arrival Thorkel asks the scientists to describe a specimen in his microscope, since his eyesight is too poor for him to do so himself. Bill identifies iron crystal contamination, much to Thorkel’s satisfaction. Then, to their astonishment, Thorkel thanks them for their services and wants them to leave.
The biologists become insulted and angry at Thorkel as they have travelled thousands of miles for nothing, so decide to set up camp in Thorkel’s stockade, insisting that he tell them more about his research.
Steve goes for a “look around” and discovers the area is rich with pitchblende, an ore of uranium and radium. Thorkel finds him and becomes angry, but as he is outnumbered, reveals he is shrinking living creatures (including a horse) using radiation piped from a radium deposit down a deep shaft…as you do!
Thorkel then invites them and his assistant Pedro (Frank Yaconelli) to examine his apparatus, but in a cruel twist that nobody saw coming he locks them inside his radiation chamber! With the information that Bill has provided, he is able to correct the flaw that has killed his prior specimens.
When his victims awaken, they find they have shrunk to twelve inches tall…