Gamera vs. Guiron stars Nobuhiro Kajima, Miyuki Akiyama, Christopher Murphy, Yuko Hamada, and Eiji Funakoshi.

While scanning the skies through their telescope, two young boys, Akio and Tom, spy a flying saucer descending into a nearby field.

The next day, the two boys — with Akio's younger sister, Tomoko, in tow — cycle to the site to investigate.

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.

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 cold, but the Space Gyaos are taking it over and the two women are the last of their kind.

Using their super-technological devices, the alien women probe the boys' minds, in the process learning about Gamera and his soft spot for children.

It is revealed that the Terran women are cannibals that plan to feed on the boys' brains in order to absorb their knowledge.

Wrapping his tail onto a monolith, Gamera throws Guiron into a canyon, causing his knife-head to be stuck.

Tom wakes up and manages to free Akio, but, during their escape attempt, unintentionally releases Guiron.

No longer under the aliens' control, Guiron rampages through the Terran city — even attacking his own mistresses as they attempt to flee to Earth.