A group of Japanese reporters discover an infant monster called a Gappa on Obelisk Island. The reporters cage the creature and take it to Japan where it becomes a media attraction. This angers the natives of the island and Gappa's full-grown parents, who head toward Japan to find their child. Its plot virtually duplicates that of the 1961 British film Gorgo.

References:
https://en.wikipedia.org/wiki/Gappa:_The_Triphibian_Monster