After a ship hits a reef and goes down near the atomic testing area, the worst is feared, but four surviving sailors are rescued, healthy and with no trace of radiation at all!
This leads to an expedition to Infant Island, part of East Biru, and it's led by the Rolithican, Clark Nelson, a slimy, unsavory character with his face twisted in a sneer. Japanese members of the expedition include Dr. Harada, the linguist and ethnologist Dr. Chujo, and a reporter named Vin Chun, nicknamed the Bulldog because he never lets go of a story. They discover a pair of foot-high grown girls and decide to leave them in peace. However, the greedy Nelson kidnaps them for his personal gain. This leads to Mothra awakened to rescue them. Despite entreaties by Chujo, Vin Chun, and his colleague Michi, Nelson refuses to let them go and puts them to work in The Secret Fairies Show, which plays to packed houses.
Compared to others in the Toho monsters canon, the production values are more topnotch than later entries, in regards to the brass band and the crowd seeing the expedition ship off, as well as the scenes of destruction. However, there are some shots where it's clear that some of the tanks are toys, as the soldiers on them are clearly plastic soldiers, and there is a scene when Nelson seizes the twins and it's clear he's holding dummies in his hand. The moth version of Mothra is a visual triumph, with its colourful markings. A pity that for the majority of the movie, we only see a dingy brown caterpillar smashing up Tokyo.
But the charm of this movie clearly go to the twin fairies, played by the Peanuts, those twin Japanese pop singers of the 1960's, Yumi and Emi Ito. They spend the majority of the movie chanting for Mothra, wearing kitschy costumes, and just overall being cute.