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.
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.
- Nobuhiro Kajima as Akio
- Miyuki Akiyama as Tomoko
- Christopher Murphy as Tom
- Yuko Hamada as Kuniko
- Eiji Funakoshi as Dr. Shiga
- Kon Ohmura as Kondo
- Edith Hanson as Tom's mother
Gamera vs. Guiron was the fifth film in the Gamera series.
Gamera vs. Guiron was released in Japan on 21 March 1969. The film was never released theatrically in the United States. It was released directly to television by American International Television in 1969 as Attack of the Monsters. The film was followed by Gamera vs. Monster X.
From retrospective reviews, AllMovie referred to the film as "one of the lesser Gamera entries" noting a smaller budget, the film's recycling of scenes from previous films, and "some atypically shoddy rear-projection effects." The review described Christopher Murphy as "a terrible actor." Though asserting that "completists may still want to check [Gamera vs. Guiron] for a few reasons," such as Guiron's "unique design" and the film's Hansel & Gretel storyline, "Ultimately, the mix of saccharine kid-movie elements and often surrealistic monster movie shocks will not be to everyone's taste"