A Star Ranger uncovers and foils a Martian plan to blow up the Earth and then shift the planet Mars into Earth's orbit to get the benefit of Earth's superior climate. Leonard Nimoy (Mr. Spock) plays a zombie henchman to the villian.

Larry Martin, member of a government defense organization, detects a strange interplanetary rocketship as it lands on Earth. Donning his jet-propelled flying suit, he investigates the remote landing site and learns that two Martians, Marex and Narab, have arrived as the vanguard of an invasion force. With the assistance of Roth and Shane, two Earth criminals, the Martians plan to construct a hydrogen bomb that will blow Earth out of its orbit. This will make room for their own dying world, Mars, which wil be moved into Earth's vacant orbital path, improving the Martian atmosphere and climate.

The Martians and their henchmen conspire to steal an array of instruments and scientific components vital to their plan, bu they are stymied again and again by Larry and his assistants Bob Wilson and Sue Davis. The Martians operate from a base located in a cave accessible only through an underwater passage. While tracking uranium ore stolen by the Martians, Larry and Bob discover the Martians Lair and are almost killed in a struggle with the aliens, who have the ability to remain underwater longer than humans.

The Martians set a timing mechanism on a detonator that will explode the H-Bomb, hurling Earth out of Orbit and into space. The escape in their rocketship, but after a ray gun battle with Larry, the ship crashes and all of its occupants are killed except Narab, who reveals the location of the bomb to Larry just before he dies. Racing against time, Larry flies to the cave, entering the Martian headquarters through the underwater tunnel, and sve the Earth by disarming the bomb seconds before it was set to explode.