Heading an aviation company that develops atom-powered motors, Jim Fairfield (Pierre Watkin) learns from his chief scientists, Vic Hardy (Hugh Prosser), that cosmic radiations are being employed for experiments at a point outside America. This information is told to Jim's nephew Billy Fairfield (Joe Brown Jr.
The heroic trio escape when the door to the trapped room is blown open by the force of the explosion. Bt triangulating the cosmic rays, Jack determines the experiments are taking place thousands of miles away. Deducing that Hardy will be taken there, Jack, Uncle Jim, Billy and Betty take off in a jet-propelled airplane.
Jack Armstrong saves himself and his friends by forcing the plane into a belly landing. Jason Grood (Charles Middleton), owner of the island's trading post and the brains behind the cosmic rays scheme, sends henchman Blair (Jack Ingram) to bring the Fairfield party to the trading post. Blair kills a native who was bringing food to Jack. The newcomers are held responsible.
Jack and Betty are saved from the fiery pit by Umala (Russ Vincent), a native who now realizes that his enemies are the white men working for Grood. The latter orders his henchmen to shoot Jack and his friends on sight. Blair and his fellow henchmen combined with Princess Alura natives maneuver the adventurers toward a dangerous rope-bridge spanning a chasm.
Jack and his friends land on a rocky ledge and make their way to the ground below. Meanwhile, Grood plans a fantastic demonstration to impress the natives. An aero globe, able to go far beyond the earth's atmosphere, is sent aloft. From their hiding place, Jack, Betty, Uncle Jim, and Billy realize a great scientific experiment is in progress.
As the aero globe settles in its berth, Jack and the Fairfield's flatten themselves against the walls of the pit and escape death. Princess Alura, believing that Xalta is on the side of Jack and his friends because of their miraculous escapes, urges them to leave the island. Jack insists on rescuing Vic Hardy first.
