When a giant stone statue on Wester Island is disturbed, the legendary monster Jiger appears and heads for Japan. Gamera tries to stop this new rival, only to be injured when Jiger lays eggs inside of him. As two boys in a submarine go on a dangerous quest inside of Gamera's body to save him, Jiger threatens the Expo '70 world's fair in Osaka.

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.