Lot of stuff going on here. An expedition to a volcanic island to find exotic animals for a theme park (Playmate Land) runs into some unanticipated mayhem. For one thing, the volcano is dangerously active, plus there's a bunch of natives. The natives are friendly (not before they're taken for cannibals, though). The other problem is some giant prehistoric creatures...
In that haven of sci-fi settings, the mountain cave--complete here with an ominous bubbling pool--the explorers find a giant egg hatching. They bring the hatchling critter back to the village; but the natives aren't too happy. So they do the natives the courtesy of taking it back to Japan. They should've listened to the natives though, who figure the creature's a bad omen--we can predict that the critter will have a rather larger mom who will come looking for it.
"Spending so much money over a stupid burnt lizard!" says the boss of the enterprise--a sensationalist magazine publisher--rather nonplussed that his crew have returned with such an unremarkable cargo. Much like the greedy types in King Kong, however, they see a good return on their investment in the long term. Meanwhile, the natives have to deal with both the volcanic eruption and its consequence--the gigantic bird-lizard's parents.
As predicted, though, mom and dad monsters set off for Japan, as the little monster has gotten much bigger. Why these monsters can swim as well as fly doesn't make much sense, but per Godzilla, etc., they just do. Pretty cool destruction scenes as the adult monsters get ashore (they seem to have a sort of JATO-assist capability as well). Also like Godzilla, they breathe out beams of flame to take out fighter-jets, and anything else as well. The authorities somehow figure that the creatures are susceptible to sound waves.
Like many of the later Japanese monster movies, there's an innocent subplot involving children (and some adults) who identity with the monsters in general, and especially any little ones. This is a device that becomes the theme of the very unique, but overwrought Mothra from the early '60s. In this case, there's a native boy to lead the sympathizers.
The army isn't conceding defeat; their sonic attack works, rousting the monsters from the lake they've been hiding in. But the quick exit (encouraged by a middle strike) has a devastating consequence. Thanks to what looks like stock footage, we see a 'monstrous' tidal wave, wiping out the nearby town.
More cool destruction, as the monsters search for the little one. He's airlifted by two gigantic blimps to the Tokyo airport, to be within viewinghearing range of the parents. Essentially, then, the two plot lines converge, as the expertsauthorities let the monster family reunite, correct predicting that they'll light out for their island and leave Japan alone. It's not explained if the island still exists, though, as it was last seen destroying itself.
The best thing about this movie is that it's absolutely crammed with action. Special effects in most of the Japanese monster movies are often suspect. Here the quality varies quite a bit. In short, we're dealing with (3) guys in rubbery suits stepping on model buildings, cars, and military vehicles. The authenticity of these scenes depends entirely on the look and movement of the monsters, and on the accuracy of the modeling.
Since the filmmakers haven't chosen an actual type of dinosaur (the scientist characters are confused as to what the monsters are, and they settle on the natives' term, 'Gappa') they can make it look anyway they want. With the major exception of the goofy chicken-like beak, the monsters look pretty good. The modeling is generally good with the land-based buildings and such, but the aquatic scenes suffer from the additional simulation of having miniature pools or tanks, betraying a very toyish look.
This dichotomy is more apparent in the island scenes. From afar, the island isn't much more than a plasticky thing floating in an aquarium, with some weak flames popping out of the volcano. But, once the crew land on it, it's very much a real place. Even the cave and native village, which presumably aren't natural, are done pretty well.
One issue has nothing to do with the movie per se, but with the title--what prehistoric planet? It's a good ol'-fashioned prehistoric island right there.
Review by davidcarniglia from the Internet Movie Database.