Gamera vs Guiron in a 1969 kaiju movie. It is the fifth in the series featuring Gamera, the giant turtle.
While scanning the skies through their telescope, two young boys, Akio and Tom, spy a spaceship descending into a nearby field. Stunned, they tell Akio's mother what they have seen, but she dismisses their story as childish nonsense.
The next day, the two boys and Akio's younger sister, Tomoko, bicycle to the site to investigate. Enthralled, Akio and Tom manage to sneak into the spaceship. Without warning, the ship takes off, leaving Tomoko behind.
The ship soars into outer space toward a field of asteroids, which sends the boys into panic. However, the heroic giant flying turtle Gamera 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 knife—emerges from an underground lair and attacks the Space Gyaos. After a short battle, the knife-headed creature cuts the Gyaos' head off and then retreats 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. The women 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 growing old and cold and the Space Gyaoses have targeted it for occupation. The knife-headed monster Guiron is the Terrans' last defense against the Space Gyaoses.
Barbella and Florbella capture Tom and Akio and put them into restraints. Using their advanced devices, the alien women probe the boys' minds. They learn about Gamera and his fondness for human children. It is revealed that the Terran women are cannibals that plan to eat the boys' brains. In preparation to extract Akio's brain for their nourishment, the women shave his head.
Gamera lands on Terra in search of the boys. The giant creatures fight and Gamera is seemingly defeated.
Tom manages to free Akio, but, in the process, unintentionally releases Guiron. No longer under the aliens' control, Guiron rampages through the Terran city, attacking its own mistresses as they attempt to flee to Earth. The knife-headed creature slices the spacecraft in half, mortally injuring Barbella. Florbella then kills Barbella, declaring 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. The rocket explodes, cutting 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 away.
- Nobuhiro Kajima as Akio
- Miyuki Akiyama as Tomoko
- Christopher Murphy as Tom
- Kon Ômura as Officer Kondo (aka "Kon-chan")
- Hiroko Kai as Barbella
- Reiko Kasahara as Florbella
- Eiji Funakoshi as Dr. Shiga
- Akira Natsuki as News Reporter
- Space Gyaos was not intended to be featured in the film. A new kaiju was planned, but due to time and budget limitations, the original Gyaos prop from Gamera vs. Gyaos was painted silver.
- In the AIP-TV US release, the scene where Space Gyaos is mutilated by Guiron was removed for its graphic content
- The police officer Kondo is nicknamed "Kon-chan" by the children. Chan is one of the many honorific suffixes appended to Japanese names. The chan suffix implies a familiar relationship and is considered somewhat cutesy, so is typically used when addressing a best friend or younger relative. A child addressing a police officer in such a fashion would be regarded as highly impertinent, hence Officer Kondo's huffy reaction.
- Movie Goof - The budding astronomer kids refer to the Earth as a "star" (Servo angrily keeps "reminding" them it's a planet!). This is likely due to a mistranslation of the Japanese word "hoshi", which can mean either a planet or a star (in the sense that they're both lights in the night sky).