When his failing vision no longer permits him to use his microscope, Dr. Alexander Thorkel, the world's greatest living biologist, summons two scientists and a mineralogist to his laboratory deep in the Amazon jungle. The scientists, Dr. Mary Robinson, Dr. Rupert Bullfinch, and mineralogist Bill Stockton, are joined in their expedition by Steve Baker, the owner of the mules they have rented. As soon as the scientists complete their examination of Thorkel's slides, the doctor insists that they leave his camp immediately, but they refuse without first learning the nature of his research. The discovery of a huge uranium deposit makes them even more curious, causing them to break into his lab. Just as they learn that Thorkel has solved the mystery of molecular structure and can shrink living tissue, the doctor bursts into the room and tricks his intruders, along with his servant, Pedro, into the chamber, where he then shrinks them. While he sleeps, his subjects escape, and when he awakens and recaptures Bullfinch, the doctor learns that he is gradually returning to normal size, and so kills him. The remaining four escape into the jungle, but Thorkel tracks them down with the help of Pedro's dog, and Pedro sacrifices his own life for the safety of the others. Their only hope now lies in outwitting the diabolical doctor, and while he sleeps, they steal his spare spectacles and break the lens in his one remaining pair. Awakening in a rage and unable to see with only one eye, the doctor falls to his death in the mine shaft while pursuing his victims. Months later, Mary, Bill and Steve return to normal size and civilization.