Gigamoth was the evil twin of a Mothra Larva, whom with it hatched from the same egg. Gigamoth was mutant Mothra that was malformed by radiation. Once it hatched, it fled the contaminated island on the South Seas it hatched on. It is learned that Gigamoth's radioactivity attracts Godzilla, and they start to fight near a Kanto power plant. A woman named Mana teleports inside Gigamoth and takes control of its body. Mana commands Gigamoth to fire an acidic mist, which corrodes everything in the area and burns Godzilla's skin. Gigamoth then webs up Godzilla and pushes him out into the sea. Gigamoth then goes up to the nuclear power plant and encases itself in a cocoon, seemingly trying to obtain energy from the power plant while it transforms. After Gigamoth does this, Mana stops controlling it, right when the Self Defense Force mobilizes a new squad of maser cannons. The masers fire many freezing beams at the cocoon, but the plan fails as Gigamoth comes out of the cocoon in its adult form. However, Gigamoth was not given enough time to complete its metamorphosis, and so its wings have not fully matured, making it unable to fly, and it proceeds to go to Tokyo.
Later, Godzilla faces Gigamoth for the second time in Atami. He fires his atomic breath at Gigamoth, but it doesn't get damaged by the beam, but rather absorbs the nuclear energy, making its wings fully developed. After witnessing the battle, Murakami realizes that Gigamoth must have formed from an extreme exposure to radiation, and believes that he could develop an antibody bacteria that could separate the radiation from the mutant bacteria in the cell structure and neutralize Gigamoth's powers. Mana interrupts Murakami's study and merges Murakami with Mothra, while merging herself with Gigamoth. However, Mothra refuses to respond to any of Murakami's commands and the two un-merge with them. Murakami then finishes the antibody bacteria.
Then, in the Fuji area, Gigamoth lays a few giant eggs of its own. Soon, Godzilla appears, attracted by the Gigamoth's radioactivity, and prepares to fight Gigamoth again. Before they can fight, a fleet of jets appear and unload tens of rockets which contain Murakami's recently finished antibody bacteria at the two kaiju. Godzilla is not affected by the rockets, but Gigamoth is weakened and made very vulnerable by the bacteria. Suddenly, Mothra arrives, in her Imago form and merged with Mana, ready to battle Godzilla. Mothra fires poisonous pollen that paralyzes Godzilla just as Godzilla fires his atomic breath and hits Mothra, sending her to the ground. Mothra and Gigamoth make eye contact, and Mothra goes right into Gigamoth's horn, making her bleed, which dissolves Gigamoth and its eggs. Mana hoped that Mothra contained a bit of mutated bacteria which would create a new creature which would be unaffected by Murakami's antibodies. Her plan works, as the two are covered by a bright light, from which Mothra's true form comes out of.
Mana stops controlling Mothra and appears in front of Murakami for the final time. Murakami uses this opportunity to confess his love for her and asks that she let them both join with Mothra. Mana refuses, but tells him he will meet someone like her soon. Mana then merges with Mothra again and proceeds to cocoon Godzilla. Mothra then grabs the cocoon and dives into the sea, taking Godzilla with her. Shortly afterwards, a search of the ocean is conducted, at which neither monster is found and both are presumed dead.
- Gigamoth was a prototype for Battra.