In order to defend Japan from Godzilla, the J.S.D.F. constructs Mechani-Kong, a giant robot gorilla designed to destroy Godzilla. The giant mech is launched into battle against Godzilla, utilizing its injectors to send a team of humans inside him. While the humans battle Godzilla from the inside, Mechani-Kong battles him from the outside.
Originally, Toho planned to remake their classic film King Kong vs. Godzilla, under the new title Godzilla vs. King Kong. When Turner Entertainment, who held the film rights to King Kong at the time, demanded a large sum of money for Toho to use Kong in the film, Toho attempted to use Mechani-Kong in the film instead. Turner Entertainment still demanded payment from Toho, alleging that Mechani-Kong was still too close to King Kong, so Toho scrapped the project and made Godzilla vs. King Ghidorah instead. Elements from Godzilla vs. Mechani-Kong were later utilized in the 1993 film Godzilla vs. Mechagodzilla 2.
Concept art of a giant mutant bird for the film also exists. The monster would have been mutated by radiation leaking from Mechani-Kong and would have fought Godzilla at some point during the film.
- According to Koichi Kawakita, the idea of a team of humans traveling inside Godzilla was inspired by the classic 1966 science-fiction film Fantastic Voyage. This concept was also featured in an early draft of Godzilla vs. Mechagodzilla 2.