Nikkatsu Studio was one of the six largest movie studio in Japan at the time this movie was made, but due to the proliferation of TV sets, Japanese movie industry was in a steady economic decline. Kaijyu or giant monster movies were about the only franchise that were still making good money, and Nikkatsu made an attempt to jump in to this genre with this movie.
A Japanese expedition to the south pacific island unwittingly stumble onto a newly hatched baby triphibian reptile which the natives call "Gappa", and brings it back to Japan. A greedy publishing magnate, and an amusement park promoter notices a good thing when he sees one and decides to display the baby at his park. This didn't sit well with the parents of the baby reptile, and they show up in Japan to claim their kid. Rest is stereotypical giant monster mayhem.
This movie as far as I know is the only movie that features a complete family of giant monsters or "Kaijyu" ever to be shown together. It's also a first Triphibian monster that can walk, breathe underwater, and fly. The story is bit derivative where I can see little pieces off of Gorgo, Mothra, Godzilla, King Kong etc. and not very original, but the special effects in this movie is surprisingly good for a studio's first attempt. Gappa is not just an mindless monster, but actually seems to have intelligence behind their actions. This lowers the tension of the monster's character as an engine of destruction, and the movie carries on with bit of a bore, but the overall production is good, and ranks as an average giant monster movie. You get to see a very young Yoko Yamamoto playing the star in this movie. She's still acting after nearly 50 years since this movie was made, and amazingly, still beautiful in her roles.