After a former student discovers a rich deposit of radium in a South American rain forest, the brilliant but arrogant Dr. Alexander Thorkel sets up a laboratory there and begins experimenting on miniaturization of animals. When the pupil questions the scientists ethics, he callously murders him. Faced with failing eyesight, the mad doctor enlists three other scientists to help identify the roadblock that has delayed completion of his project. After a long and arduous journey by pack mule through the Andes, they arrive at the laboratory and quickly identify Thorkel's problem. Steve Baker, a miner and fourth member of their party, realizes the value of the radium and is reluctant to leave as are the scientists, who feel slighted by Thorkel's dismissive attitude and their concern about his now growing megalomania and paranoia. Feeling threatened by their curiosity concerning the true nature of his work, he miniaturizes them along with a native servant as they become his de facto guinea pigs. Although their situation appears grave, their one hope lies in Thorkel's Achilles heel, his failing eyesight.