On an expedition to a “supposedly” uninhabited island where atomic tests have been conducted, a native tribe is discovered that worships a giant egg named Mothra (actually Mosura).  There are also 2 tiny twin girls (only 12 inches high) who sing a cool “Mo-su-ra” song, and they end up getting kidnapped, taken back to Tokyo where they perform the “Mo-su-ra” song in a show.  Turns out the song helps to awaken the egg which hatches into a giant caterpillar which swims across the ocean, destroying everything in its path, in order to rescue the girls.  Somewhere along the way, the caterpillar spins a cocoon and hatches out into a giant moth, wreaks even more havoc and eventually flies the tiny girls back home (or something like that).