After singing twin maidens are kidnapped from a tropical island, a giant larva (and soon-to-be moth) travels to Japan to rescue them.

An expedition of Japanese and Rosilican scientists visit an island in the Pacific which has been used as an atomic testing ground. They find the island inhabited by tiny tribesmen and discover foot-tall twin maidens guarding a huge egg which is held sacred by the islanders. Nelson, an unscrupulous Rosilican, later kidnaps the twins to exhibit them as freaks in Tokyo. The egg hatches, and a giant larva crawls toward the Japanese capital to rescue the women. Leaving a trail of destruction behind it, the creature finally spins an invulnerable cocoon. Atomic rays are used in an attempt to destroy the cocoon, but the tremendous heat generated speeds up the creature's metamorphosis, and a monstrous moth emerges. As it pursues Nelson to Rosilica, the monster creates powerful wind storms with its wings, destroying the Rosilican metropolis and killing thousands. Angry citizens kill Nelson; and to ensure world peace, the two governments surrender the twins to Mothra, who returns with them to their island.