A creature emerges from the moon's surface and quickly flies down to Earth. Upon landing, the creature is spotted and dubbed Redmoon. Almost simultaneously, another monster appears from Habu Island, located in the Okinawa chain. This second creature is given the name Erabus.

The Japanese Self Defense Force feels that the appearance of the two kaiju is a perfect opportunity to lead the monsters together so that they will kill one another. The plan is a success, as they are able to lure the two monsters together; however, instead of fighting the two monsters mate, as it's discovered that Redmoon and Erabus are male and female of the same species. Shortly after mating, Erabus gives birth to small creature called Hafun.

While the SDF is trying to figure out what to do with with the creatures, Hafun is kidnapped by an entrepreneur with the hopes of displaying the creature for profits. Unfortunately, Hafun dies in the process of being moved, causing Redmoon and Erabus to go berserk and begin laying waste to the cities in Okinawa. However, Godzilla arrives on the scene and manages to defeat both Redmoon and Erabus, putting an end to the destruction.

The film was planned as part of the yearlong celebration of Tsuburaya Productions' 10th Anniversary in 1972. Tsuburaya Productions approached Toho with the idea of combining elements from their new popular film Daigoro vs. Golaith (1972) with that of Toho's most popular, and Eiji Tsuburaya's most famous, creation: Godzilla. Toho gave the project the green light, and agreed to loan Tsuburaya Productions' a Godzilla suit.

Tsuburaya Productions' assembled a team for the film which consisted of Noboru Tsuburaya and Toyowaki Awa as producers, Shokei Tojo as director and Kazuo Sakawa as the Special Effects director. The Kajiu in the film, Redmoon, Erabus and Hafun where to be modified suits from Tsuburaya Productions' Daigoro vs. Golaith (1972) with Redmoon being the Goliath suit, Erabus being the Daigoro's Mother suit, and Hafun being the infant Daigoro suit. Reportedly, the Godzilla suit for the movie was going to be the one from Son of Godzilla (1967), probably so that Toho could continue to use the Destroy All Monsters (1968) suit in their own productions. Unfortunately, the project never saw the light of day.