Following reports of human life on Infant Island, the supposedly deserted site of atomic bomb tests, an international expedition to the heavily-radiated island discovers a native tribe and tiny twin female fairies called “Shobijin” who guard a sacred egg. The overzealous expedition leader kidnaps the Shobijin to exhibit in a Tokyo stage show but soon they summon their protector, hatching the egg and releasing a giant caterpillar. When Mothra arrives in Japan and transforms into her final form, the nation and its people face their destruction.