The storyline was that a giant serpent like monster, called Garasharp, appeared and started destroying Tokyo. Gamera soon appears and defeats Garasharp, even though the monster has a large array of weapons including poisonous breath and a sonic rattle. But then, two baby Garasharps crawl out of the carcass. The military tries to kill them, but Gamera lets them crawl onto his back and he flies them to a deserted tropical island.
Also appearing in the film would the monster Marukobukarappa. The role/purpose of the monster was never revealed but it is believed that it would've fought Gamera as well.