Shortly after conception, the film was reworked into Godzilla vs. Mechagodzilla, with King Caesar replacing Mothra, and Mechagodzilla replacing Garugan. The Garuga Aliens were also reworked into the Black Hole Planet 3 Aliens. It is also possible that the Garuga Aliens were adapted from the Garogas, a race of aliens which are the main antagonists of Zone Fighter.
Okinawa is threatened by the arrival of a mysterious alien race, the 'Garuga Aliens', who proceed to unleash their vanguard Kaiju Garugan on Okinawa. Godzilla, Mothra and Anguirus travel to the city to defend it, but even in a three-on-one fight, the battle is not easy.