In his laboratory in South America, The Doctor Thorkel comes to make a revolutionary discovery. Thanks to Radium energy, a kind of uranium he mines from a draw next to his laboratory, he now can reduce things. To concretize his experiences, Thorkel must be helped by his colleague, the Doctor Bulfinch, a famous biologist, who accepts and gather his assistant Mary Mitchell, the mineralogist Bill Stockton and Steve Baker for a long expedition of 16000 km to join the scientist.
Because his view does not cease to decline, Thorkel needs the works of Bulfinch on the molecular tissue structure. The expedition arrived, Thorkel uses the eyes of Bulfinch in order that the former confirms via a microscope that Thorkel has indeed found the link lacking of his experiences, then throw out the scientists without other explanation.
Vexed to have traveled so long to be thrown like this, the scientists ask Thorkel to delivers them the results of his secret experimentations. But the former does not drop anything, while from his secret laboratory, he continues demoniac experiences and succeeds to miniaturize a horse while keeping it in good health.
But the too curious scientists, break-in in the laboratory of the learned madman that surprises them. After some explanations, Thorkel finishes by yielding and explains them the nature of his experiences. But it is a trap and he encloses the small crew in an experimental room that reduce them until reaching some thirty centimeters tall.
Scientists are now surrounded by an hostile world that is no longer adapted to their dimensions. As they refuse to cooperate with Thorkel, this last captures the Doctor Bulfinch and begins on him some observations. He observes that the small man resumes to grow little by little. Not wishing him to explain his colleagues this news, Thorkel kills Bulfinch, while the others escape the laboratory and leave to find refuge in the jungle...
Spoiler ! the end of the story is revealed below…
But Thorkel pursuit them. He can not allow them to find back their original seize, and kills Baker with a gunshot. The others flee and hide in the fishpond of the scientist who, without noticing it, returns them in his laboratory. Decided to kill Thorkel, they steal all glasses of the completely myopic scientist depriving him from his vision and attract him in the radium draw where he falls.
Some months later, Mary Mitchell and Bill Stockton have resumed naturally their original size. They finally leave South America well decided to never unveil what they have lived there.