Earth is invaded by evil aliens from another galaxy. Their attack is stopped by the mighty force of prehistoric monster Gamera. The aliens scan Gamera's brainwaves to discover and exploit his only weakness. By taking two young boys hostage, they are able to seize control of the monster and use his powers against the Earth. As Gamera ravages the cities of Japan, the two children struggle to escape, freeing him from the the aliens' command and provoking a deadly final battle between space monsters and Earth's colossal defender.