Unusual volcanic activity in Japan awakens Gyaos, a bloodthirsty flying monster with the power to slice things in half with an ultrasonic ray. While scientists and the military scramble to devise a way to stop this new threat, a young boy forms an alliance with Gamera; a monster no one else seems to trust.

Gamera escapes from his rocket enclosure and makes his way back to Earth as a giant opal from New Guinea is brought back to Japan. The opal is discovered to have been an egg that births a new monster called Barugon. The creature attacks the city of Osaka by emitting a destructive rainbow ray from his back, along with a freezing spray capable of incapacitating Gamera.