With the box office success of King Kong vs. Godzilla and Mothra, Toho chose to send Godzilla against the butterfly in a movie that was meant for children instead of adults. It’s also the last movie — until the Heisei era — that Godzilla would be against humanity.
As a typhoon leaves behind great damage, a bluish-gray object has been left behind as well as a giant egg which is taken by Kumayama, the owner entrepreneur of Happy Enterprises. He decides that science will have nothing to do with the egg. It’s time to make money off it.
That’s when the twin Shobijin arrive and explain that the egg belongs to Mothra and if it hatches, Mothra’s larva child will destroy Tokyo as it looks for food. The Japanese government begs them to send Mothra to stop Godzilla, who has come back for the strange object left behind, one that is emitting radiation. Despite all the outside world has done to their island and even though Mothra is in great pain and dying of old age, they decide that they must help.
While Godzilla does destroy Mothra with his atomic breath, her twin children arise in their larva form and spray the King of the Monsters repeatedly with their silk and allow Godzilla to be captured.
Henry G. Saperstein acquired the American theatrical and TV rights. He planned on the name Godzilla vs the Giant Moth, but American-International Pictures bought the movie and released it as Godzilla vs. The Thing, censoring Mothra from the poster to build audience excitement for who the big green lizard would fight. After so many of their films being released in America, Toho shot footage specifically for export, such as a scene where U.S. troops help the Japanese fight the monsters.
When everyone arrives on Infant Island, the skeleton of a turtle can be seen in the background. This character, known as the Mystery Bones of Infant Island, is a living kaiju that was inspired by Mondo Cane.