When an alien spaceship is spotted rocketing toward Earth, Mr. Steele, Larry Martin, Bob Wilson and Sue Davis, members of the Interplanetary Patrol, spring into action. The ship, manned by Martians Marex and Narab, lands in a remote desert area where it is met by Roth and Shane, who work for scientist Dr. Harding. After loading equipment into Roth and Shane's truck, Marex orders the pilot to take off. As Narab, Marex, Shane and Roth drive away, Larry, who has flown there in his rocket suit, spots their truck and lands on top of it. When Roth tries to dislodge Larry by wildly swerving the truck, Larry attaches a radio device to the top and jumps off. Marex then instructs Shane and Narab to drive to a secret cave and unload the equipment while he and Roth proceed to Harding's lab. There Marex discloses that he knows Harding has been selling atomic secrets to an unfriendly power and threatens to expose the doctor unless he agrees to help the Martians. After explaining that climatic changes have necessitated changing the orbit of Mars, Marex states that he plans to detonate an H-Bomb explosion strong enough to send the Earth spinning out into space, thus allowing Mars to take Earth's place in the galaxy. To accomplish this, Marex plans to assemble a bomb on a small island near the cave. Meanwhile, at the cave, Narab transports the components of the bomb down a ladder to an underwater tunnel that leads to the island. Needing uranium to complete the bomb, Marex orders Shane and Roth to rob a train carrying a shipment of uranium as it stops at the town of Peralta. Soon after, Larry, who has followed the radio signal to the cave, finds a charred piece of a map in the deserted cave. At the Interplanetary lab, Larry uses infrared to decipher that the town of Peralta has been underlined on the map. Larry and Bob split up to search the area, and when Larry reaches Peralta, he sees Roth and Shane battling Bob on the top of the train as it pulls out of the station. As the thugs overpower Bob and handcuff him to the top of the train, Larry commandeers an abandoned tank and pursues them. When the tank overtakes the train, Roth and Shane jump off, and Larry climbs onboard to free Bob. Soon after the Martian craft is spotted in the area, Larry and his pilot Dick take off in an Interplanetary rocket to investigate. Larry zeroes in on the rocket just as it is taking off from a lake. However, when the Martians fire their weapons and damage the fuel line of his rocket, Larry is forced to return to the airfield. Suspecting that the spaceship delivered some cargo that was then picked up by a boat, Larry drives to the boathouse to investigate and finds electrical equipment stacked on a speedboat. When Larry steals the boat, Roth and Shane pursue him toward a dam, forcing him to jump overboard before the speedboat and its equipment plunges over the side. Knowing that their quarry will have to replenish the lost shipment, Larry receives word that two men are buying an unusually large amount of electrical equipment. He and Bob drive to the electrical store just in time to follow the thugs' truck to a shed where they load a robot into their truck and speed away. When news comes that a robot has robbed a bank and the getaway car is headed into the hills near the cave, Larry rockets down to have a look. While Narab works in the island, Larry enters the cave and arrests Roth and Shane. Hearing the confrontation over his radio transmitter, Narab signals the robot to attack Larry. As Shane and Rob escape, Larry disables the robot with an ax, then radios Bob for help. Noticing the ladder leading from the cave to the water below, Larry decides to take the robot back to the lab and then return with diving gear to investigate. To prevent Larry from locating the trapdoor leading to the entrance of the island, Marex and Narab plant explosives in the tunnel. Upon returning to the cave, Larry descends the ladder while Bob stays above ground. When Roth and Shane suddenly appear and shoot at Bob, Larry hears the gunshots and climbs back up the ladder, avoiding the brunt of the explosion. Roth and Shane escape, and later, Marex arranges for them to hijack a truckload of uranium. While Shane hides in the back of the truck, Roth, posing as a guard, orders the driver to get out of the truck. They then drive the uranium to the cave. When the Martian ship delivers the detonator, Harding makes the final calculations necessary to detonate the bomb. Meanwhile, at the Interplanetary lab, the robot has been repaired, enabling Larry to trace its control panel beam to Harding's house. Instructing Bob to remain at headquarters and listen through a radio transmitter, Larry drives to Harding's house where Harding knocks him out and flees. Hearing the altercation, Bob puts the robot in the backseat of his car and drives to the doctor's house where he picks up the now-conscious Larry. At the cave, Harding, Marex and Narab climb down the underwater tunnel to the island to complete assembly of the bomb. When Bob and Larry arrive at the cave, they activate the robot, which chases Roth and Shane over a cliff. With two hours before the bomb is set to detonate, Larry locates the trapdoor to the island and sends for reinforcements. With their escape route blocked, Harding panics and tries to shut off the detonator. After shooting the doctor, Marex and Narab climb back into the cave from which they throw gas bombs, driving Bob and Larry outside for fresh air. Hurrying to their rocket ship hidden under some branches, they take off, and Larry radios Dick to meet him with the Interplanetary rocket. Larry fires the rocket's ray gun at the Martian craft, sending it crashing to earth. Narab, the lone survivor, warns Larry about the bomb, and with only seconds to spare, Larry dives down the tunnel and disconnects the detonator.
