A volcano erupts in Japan. Gamera appears, sucking up heat energy from the lava. He gets so greedy that he eventually jumps directly into the mouth of the volcano! Hmm, why would he be so desperate to soak up that much energy? Could it be that he thinks he'll be going into combat soon? Given that the earthquake that started the eruption in the first place opens up a fissure nearby, thereby unleashing a giant bat monster that had been buried there for who knows how many eons, I'd say Gamera's instincts are right on the money.
The first battle does not go well for our heroic turtle. The bat monster, which this movie's Kenny dubs "Gaos", kicks Gamera's ass with ease. Gamera almost loses an arm to Gaos's sonic beam! The rematch that occurs a bit later doesn't go very well, either. Gamera tries to stay inside his shell and ram Gaos to death by flying into him at great speed but, come on, Gaos is a BAT. Gamera is a TURTLE. If there's one place a bat has the advantage over a turtle, it's in the air. The fast and maneuverable Gaos makes Gamera look like a fool during their aerial battle.
Since Gamera got his clock cleaned by Gaos twice, humanity realizes it may actually have to get off its collective ass and try to stop a monster itself for a change. To this end, the Old Scientist character constructs a "blood generator" (!?) on a rotating platform. You see, Gaos drinks blood and is highly susceptible to dizziness. (So says Kenny, anyway. He must be right because he's been keeping a journal about Gaos and everything.) The idea is that Gaos will be lured to drink the blood, then rendered woozy and helpless by the rotating platform, and will thus be unable to return to his lair before the sun comes up. Sunlight is highly detrimental to Gaos, you see. He lost a couple of toes during Round 2 with Gamera, and they began to sublimate as soon as the sun's rays touched them. Naturally, this highly sensible plan fails. Turns out Gaos is a lot heavier than the engineers who built the rotating platform anticipated; the mechanism fails allowing Gaos to recover and flee just before sunrise.
Well, it's all up to Gamera, now. Gamera pulls out all the stops for Round 3, which leads to an absolutely brutal monster battle. Gamera gets sliced to ribbons by Gaos's sonic beam and poisoned by the toxic mist Gaos emits from his nipples. Gaos gets his wings shredded and his neck brutalized by Gamera's tusks. The fight ends when Gamera realizes that there is exactly ONE place he indisputably has every advantage over Gaos: the inside of an erupting volcano!
This movie makes me even more curious about something I'd begun to suspect while watching Gamera vs. Jiger: Has Gamera has fought these monsters before? In Jiger, Gamera obviously knew that Jiger was buried underneath that whistling statue, since he tried to prevent the digging crew from moving it. In this movie, Gamera seemingly knows that the earthquake is going to free a powerful monster, and he goes so far as to dive into molten lava in order to supercharge his power. He clearly has foreknowledge of Gaos's sunlight weakness, too. Can it be that, sometime way back in prehistory, Gamera - or his ancestor - fought them and was responsible for entombing them in the first place? I really want to think so, because that's wicked cool.
Gaos is Gamera's most powerful foe by a long shot. He outfights Gamera on the land and in the air. He is stronger than Gamera, faster than Gamera, more mobile than Gamera, and his sonic beam and poison dust are much more effective weapons than Gamera's fire breath. Not only that, Gaos has such a rapid healing factor that he heals minor damage while Gamera fights him! Even the loss of a limb is no big deal, requiring only a few hours of healing time to regenerate. His vulnerability to sunlight really is the only weakness Gamera can exploit. It isn't often the hero monster is utterly outclassed by his opponent in a kaiju flick. Gaos is a great looking monster, too. With his sleek, angular build, he looks more like a living fighter jet than any sort of animal. It's no wonder that Gaos was the first monster Daei used when they rebooted the Gamera series in the '90s.
Note: the shot of Gamera nearly submerged in Tokyo bay, hanging on to Gaos's foot, struggling to hold him until the sun rises, is one of the coolest images to be found in any movie of 1960s vintage.