Originally intended to be a solo film about Jet Jaguar, They quickly re-wrote the script to add Godzilla and Gigan once they realized that Jet Jaguar couldn't carry the film alone and shot the film in just 3 weeks. The rushed shooting schedule and script that was made up on the fly caused a massive drop off in quality, Resulting in this movie being one of the worst in the series.
Review of Godzilla vs. Megalon
