Two curious boys, Akio [Nobuhiro Kajima] and Tom, scan the night sky with a telescope and glimpse a flying saucer descending into a nearby field. They rush to tell Akio's mother, who writes off their story as childish nonsense. The next day, the two boys — with Akio's younger sister, Tomoko [Miyuki Akiyama], in tow — cycle back to the site to investigate more closely. They manage to slip into the ship, but it suddenly launches away, leaving Tomoko behind. It shoots off toward a field of asteroids, sending the boys into a panic. Yet Gamera [Umenosuke Izumi] appears and clears a safe path for the craft. The ship, traveling at near light speed, leaves Gamera behind and transports the boys to an unknown planet, where it lands on the outskirts of an alien city.
A silver “Space” Gyaos ominously appears, menacing the ship and the two children. Before the creature can attack, a second, bizarre monster — its head blade-shaped like a Bowie knife — emerges from an underground lair and attacks Space Gyaos. The Space Gyaos fires a beam that bounces off the knife-headed creature’s blade and severs its own right leg. After Space Gyaos tries to retreat, the knife-headed monster lunges, chops off Space Gyaos’ left wing, and then its right wing. It then decapitates Space Gyaos and brutally tears the body into pieces before retreating to its lair.
Akio and Tom explore a portion of the alien city and meet Terra’s two inhabitants: Barbella [Hiroko Kai] and Florbella [Reiko Kasahara], who explain that their planet Terra orbits the sun directly opposite Earth, which is why Earth’s astronomers have never discovered it. Terra is facing extinction; not only is the planet growing cold, but the Space Gyaos are taking over, and Barbella and Florbella are the last of their kind. The knife-headed monster, called Guiron by the Terrans, is their final defense against the Space Gyaos.
Barbella and Florbella suddenly incapacitate Tom and Akio and put them into restraints. Using their super-technological devices, the alien women probe the boys’ minds, learning about Gamera and his soft spot for children. It is revealed that the Terrans are cannibals who plan to feed on the boys’ brains to absorb their knowledge. In preparation to extract Akio’s brain for nourishment, the women shave the child’s head. On a rescue mission, Gamera lands on Terra in search of the boys. The women unleash Guiron to attack the giant turtle. Guiron plans to cut Gamera in half, but Gamera grabs one of Guiron’s front legs and bites into it. Guiron tries to shake off the towering tortoise, wrapping its tail around a monolith, but Gamera throws Guiron into a canyon, causing the knife-headed creature to get stuck. Gamera uses his flame breath on Guiron. Guiron fires his shurikens, injuring Gamera’s cheeks. Gamera searches for healing, grabbing ice-like boulders. Guiron fires more shurikens, and Gamera counters by using a long boulder to ricochet the shurikens back into Guiron’s own body. Guiron staggers away, while Gamera tumbles into a lake unconscious and on his back.
Tom wakes up and frees Akio, but, in the escape, inadvertently releases Guiron. No longer under the aliens’ control, Guiron rampages through the Terran city — even attacking his former mistresses as they try to flee to Earth. The knife-headed creature slices the spacecraft in half, mortally wounding Barbella; Florbella kills Barbella as she laments that useless members of their society are euthanized. While Guiron attacks the base where the boys are imprisoned, Gamera awakens and renews his assault on the alien creature, ultimately ramming Guiron’s head into the ground. Florbella attempts to flee in a rocket, but the boys launch missiles that destroy it, and she dies as a result. Gamera catches one of the missiles and spears Guiron into his shuriken base. He then uses his flame breath on Guiron, igniting the missile and cutting Guiron in half. Gamera welds the damaged spacecraft back together with his flame breath and carries the ship, with the two boys aboard, back to Earth.
Back on Earth, the boys are returned to their mothers, and they all say goodbye to Gamera as he flies off into the sky.
