|Date of Release:||April 12, 1940|
|Boogeyman:||Dr. Alexander Thorkel|
Three scientists — mineralogist Dr. Bill Stockton and biologists Dr. Mary Robinson and Dr. Bullfinch — are summoned by Dr. Alexander Thorkel to his laboratory in the Peruvian jungle. They are accompanied by a local miner, Steve Baker who suspects that Thorkel is secretly mining ore. When they arrive they find Thorkel has brought them all the way there only to identify some crystals under his microscope, since his eyesight is too poor for him to see them himself. Their job done, he now wants them to leave.
Insulted, they set up camp in Thorkel's stockade until he will tell them more about the experiments in which he is engaged. They discover that Thorkel is attempting to shrink living organisms using radiation. When he finds them snooping in his laboratory, he angrily locks them (along with his assistant Pedro inside his radiation chamber. When they emerge they find they have shrunk to twelve inches tall.
At first willing to play-act the role of benevolent despot with his miniaturized captives, Thorkel reveals the more sinister side of his personality by abruptly murdering Bullfinch in cold blood because the effects of the ray are wearing off. The rest of the picture details the escape efforts of the four pint-sized protagonists as they hack their way through a jungle of gigantic foliage and do battle with oversize wildlife. By morning, they attempt to cross a river via small boat but are interrupted by an alligator. Thorkel then comes in, killing Pedro and igniting the grass to kill the rest. The fugitives are able to hide in one of Thorkel’s specimen cases and are unknowingly brought back to his lab.
While Thorkel goes to check on an apparatus he has set above an old mineshaft, Bill, Steve and Mary prepare to kill him with his own shotgun when he lies in bed. However, he instead falls asleep at his desk. Steve then attempts to steal Thorkel's glasses, without which he is nearly sightless. Accidentally alerted, Thorkel chases the shrunken trio to the mineshaft and nearly slips. The heroes take advantage of the distraction to cut his rope, causing Thorkel to plunge into the pit.
Months later, Bill, Steve and Mary are seen returning to the South American town, restored to their original size. Bill and Mary are in love.