The Garuga Aliens, using their monster Garugan, lay siege to Earth. It's up to Godzilla, Mothra and Anguirus to stop the assault, resulting in a climatic final battle in Zanpamisaki, Okinawa.

This was the original concept for the 20th anniversary film for the Godzilla franchise. The story was created by Shinichi Sekisawa and Masami Fukushima, while the screenplay was developed by Hiroyasu Yamaura and Jun Fukuda and was submitted in 1973. The concept continued the trend of aliens using a monster to try and conquer Earth. It was also the second attempt to try and create an entry in the Godzilla series that took place in Okinawa after the scrapped Godzilla vs. Redmoon. Giant Monsters Converge on Okinawa! Showdown in Zanpamisaki was also going to be the debut film of the Nami character, best known as the princess who awoke King Caesar by singing to him. If she was going to play a similar role with Mothra in this production, or something else altogether, is unknown.

Eventually, this idea was taken back to the drawing board for a heavy reworking. In its next phase of script writing, Mothra and Garugan had been replaced by King Barukan and Mechagodzilla respectively while this new draft was titled Showdown in Zanpamisaki: Godzilla vs Mechagodzilla. The Garuga Aliens were also replaced with the Black Hole Aliens, while the disguise aspect of Mechagodzilla was added in. In terms of King Barugan, the creature was a crimson and bronze colored monster that was 50 meters tall, weighed 30,000 tons, boasted prism reflective eyes and horns that were said to deliver the final blow to his enemies. During the concept phase, the horns were dropped and the name was changed to King Caesar, leading to the final version of Godzilla vs. Mechagodzilla (1974).

As an aside, its worth bringing up the Baragon character, due to the fact that it's often believed that the monster was at one time supposed to be in Godzilla vs. Mechagodzilla (1974). This belief originated from Anguirus' odd burrowing behavior, and the fact that Gorosaurus demonstrated a similar out of character ability when he replaced Baragon for much of Destroy All Monsters (1968). However, this role had always belonged to Anguirus, even long before the Mechagodzilla character had even been thought up.

Note: No pictures of the Garugan exist. The above image is simply Toho Kingdom's interpretation of the monster.