Mothra's twin nymphs and children from the city find a lost city, as well as a giant monster that is attracted to environmental calamities.

Three kids join Mothra's priestesses on a journey to find the Lost Treasure of Nilai Kinai, an ancient lost "under-the-sea" civilization in hopes of preserving the Earth's declining environment. Pollution has unyielded the evil monster Dagahra, a monster seemingly too powerful for Mothra to vanquish. As a result, the environment's fate lies in the hands of the Lost Treasure.—Oliver Chu