While scanning the skies through their telescope, two young boys, Akio and Tom, spy a spaceship descending into a nearby field. Stunned, bewildered, and bemused, they tell Akio’s mother what they have seen, but she dismisses their story as childish nonsense. The next day, the two boys — with Akio’s younger sister, Tomoko, in tow — cycle to the site to investigate. Enthralled, Akio and Tom manage to steal into the spaceship. But then, without warning, the ship takes off, leaving Tomoko behind. It soars into outer space toward a field of asteroids, which sends the boys into panic. However, Gamera (obviously aware of the boys’ plight) appears and clears a path for the ship through the asteroids. The spaceship, flying near the speed of light, leaves Gamera behind and transports the boys to an unknown planet, where it lands on the outskirts of an alien city. Suddenly, a silver “Space” Gyaos appears, menacing the ship and the two young boys. Just before the creature attacks, a second, bizarre monster — whose head resembles a ginzu knife — emerges from an underground lair and attacks the Space Gyaos. The Gyaos emits a beam that reflects off the new creature’s blade-shaped head and cuts off its own right leg. After the Space Gyaos attempts to retreat, the knife-headed creature lunges and chops off the Gyoas` left wing, before cutting off his right wing. The creature then cuts the helpless Gyaos’ head off and brutally cuts the body into smaller pieces before retreating back to its lair.
Akio and Tom explore a portion of the alien city and meet the planet’s only inhabitants: two beautiful women, named Barbella and Florbella, who explain that their planet, known as “Terra,” orbits the sun directly opposite the earth, which is why it has never been discovered by earth’s astronomers. Furthermore, Terra is facing extinction; not only is the planet growing old. The knife-headed monster, which the Terrans call “Guiron,” is their last defense against the Space Gyaos.
Barbella and Florbella suddenly turn on Tom and Akio and put them into restraints. Using their super-technological devices, the alien women probe the boys’ minds, in the process learning about Gamera and its soft spot for human children. It is revealed that the Terran women are cannibals that plan to feed on the boys’ brains. In preparation to extract Akio’s brain for their nourishment, the women shave the child’s head. On a rescue mission, Gamera lands on Terra in search of the boys. The women deploy Guiron to attack the giant turtle. Guiron planned to sever Gamera in half, but Gamera grabs one of Guiron’s front legs and chomps onto it. Guiron tried to shake off the towering tortoise. Wrapping his tail onto a monolith, Gamera throws Guiron into a canyon, causing his ginzu-head to be stuck. Gamera used his flamethrower on Guiron. Guiron used his shurikens to penetrate Gamera’s cheeks. Gamera tried to heal his wounds by grabbing ice-like boulders. Guiron used his shurikens again. This time, Gamera used the longest boulder to ricochet the shurikens into Guiron’s own body. Guiron trudges away while Gamera tumbles into a lake unconscious and on his back.
Tom manages to free Akio, but, in the process, unintentionally releases Guiron. No longer under the aliens’ control, Guiron rampages through the Terran city — even attacking its own mistresses as they attempt to flee to Earth. The knife-headed creature slices the spacecraft in half, mortally injuring Barbella; Florbella kills Barbella as she relates that useless members of her society are euthanized. While Guiron attacks the base where the boys are imprisoned, Gamera awakes and renews his assault on the alien creature, ultimately ramming Guiron’s head into the ground. Florbella attempts to flee on a rocket, but the vehicle is sliced in half by Guiron and she dies as a result. Gamera catches half of the rocket and spears Guiron into his shuriken base. Gamera uses his flamethrower on Guiron where the rocket was; the rocket explodes, severing Guiron in half. Gamera uses his flame energy to weld the alien spacecraft back together and carries the ship and the two boys back to Earth. On Earth, the boys are returned to their mothers and they all say goodbye to Gamera as he flies off into the night.