Watch Jack Armstrong

Jack parachutes to safety as the Are globe explodes, killing Zorn and other Grood henchmen and destroying the master death machine. Regaining consciousness, Vic Hardy demands that Grood sign a full confession of his guilt. Jack, Uncle Jim, Billy, and Betty enter the secret laboratory just as Grood overpowers Hardy. Grood escapes to a secret airfield where a plane is waiting.

Struggling with one of the guards, Jack had fallen to the ground before the truck went over the cliff and destroyed the infamous Beam Gun. Grood learns from Zorn that a master Cosmic Beam Annihilator has been completed and installed on the Aero globe. He plans to annihilate an entire city. Hardy is to go along on the flight where Grood plans to kill him.

The figure in the path of the death ray was Pierce and he was killed, Grood's henchmen capture Jack and Uncle Jim and take them to the laboratory. But Vic Hardy is there, and he knocks out the guard. Hardy remains behind, hoping to circumvent Grood's scheme for world domination. Jack and Uncle Jim, joined by Billy, go to Pierce's mill.

Jack, unharmed by the fall, escapes his opponent, commandeers Grood's truck and rescues Uncle Jim and Billy. They cannot carry off the Beam Gun, but take the activator tube, an important part of the weapon. Pierce starts out for the underground laboratory to find a substitute for the activator. Jack and Uncle Jim also head there determined to smash Grood's diabolical scheme to dominate the world.

The beam is deflected when Jack and Billy attack and overpower the henchmen operating the Annihilator. Pierce and his men, now opposed to Grood, wrest the gun away from Jack, but they cannot operate the Annihilator gun without solarium, an important element needed for its operation. Grood and his henchmen proceed to Pierce's mill headquarters.

The knives fly wild when two shots ring out. Uncle Jim arrived with Billy and Umala just in time. But they are surrounded by hostile natives After a fight between Umala and Naga (Frank Mario), Princess Alura's traitorous prime minister, with Umala victorious. the white men are freed, and Princess Alura resumes her royal throne.

Jack and Uncle Jim who have been trailing the truck are set upon by Grood's henchmen. The timely arrival of Umala and friendly natives saves the situation' Billy is rescued. Grood now plans to eliminate Umala and his followers for their aid to Jack and the Fairfield's. He attempts to arouse the natives through the voice of Xalta (his own method for controlling the island.

Betty reaches the well just in time to save Jack, Uncle Jim, and Billy. Uncle Jim then decides to confront Grood, now suspected to be the brains behind the terrible scheme, and intimates a knowledge of the secret caverns where experiments are in progress. Grood refuses to be drawn out. Bill then hides in a packing case at the trading post, hoping to be taken to the secret laboratory.

Jack retains consciousness long enough to douse the poison gas bomb with sand, saving himself and Uncle Jim. Later, Pierce joins them, still pretending friendship, to look for Vic Hardy. They are trailed by Blair who, when questioned at length, gives them false directions to Hardy's location. Pierce radios Zorn, using a miniature transmitter, that the party will pass near a secret well.

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.

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.

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 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.

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.

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.