Gamera helps the boys to escape, but he remains trapped in the force field while the aliens scan his memories.

Powerless to stop them, Gamera surfaces and follows the UFO ashore, whereupon a mind control device is attached to the back of his head.

The boys help Gamera break free from the mind control device and succeed in escaping from the spaceship.

Under the leader's command, each of the aliens merge to form the giant monster Viras, who immediately engages Gamera in battle.

No American child actors who spoke Japanese could be found at the time of production, so Carl Craig, the 11-year-old son of a US military officer stationed in Japan, was cast.

[1] It was "such a big success in Japan" that the studio asked director Yuasa if he could produce two additional Gamera films per year.

[1] In a retrospective review, AllMovie stated that the film offers "everything one expects from a massive zero-budget monster battle film" while also having "unintentionally brilliant quirky moments like the hypnotized army, Japanese men with glowing eyeballs, and a fiendishly fake-looking squid alien who wishes to attack Earth for its supply of nitrogen.

"[7] American writer John LeMay said that "in terms of character utilization and script, Gamera vs. Viras could be the film where it all came together.

"[5] Filmmaker Hayao Miyazaki found the film memorable as a child but found it absurd for humanity to surrender to aliens for the safety of two boys,[8] while Gamera vs. Viras and Miyazaki's Future Boy Conan bear similarities in plots as both were based on The Adventures of Tom Sawyer.