This 1968 film was originally called ガメラ対宇宙怪獣バイラス, Gamera tai Uchu Kaijū Bairasu (Gamera versus Outer Space Monster Bairasu), but is also known as Gamera vs. Viras, and Destroy All Planets.
Aliens called the Virans try to invade Earth by sending a spaceship to attack it, but this attcked is stopped by Gamera, who destroys the attacking craft. The Virans send a second spaceship and implant a device on the back of Gamera's head so they can control him. They decide to use Gamera to destroy Japan! (I bet you didn't see that coming....)
As a last resort, all the Virans merge into one giant squid-like monster called Viras (hence the title). They fight it out, but Gamera flies his enemy up into the stratosphere which suffocates Viras.
The effects in this effort are below par, even for Gamera. They are just plain old bad.
There have been 13 Gamera movies; this is number four in the series. Constant readers will already be aware of my feelings towards giant flying turtles with flames coming out its ass. This film does nothing to alter my perception.