Mothra vs. Bagan was proposed in 1990 by Tomoyuki Tanaka. It would have been Mothra's first film since Destroy All Monsters and would have introduced the monster Bagan, who had previously appeared (in a vastly different form) in the scrapped first draft of The Return of Godzilla. The film would have, on an interesting note, included the character of Miki Saegusa (who had appeared only in Godzilla vs. Biollante at that point), and would have tied into the preexisting Heisei Godzilla films. Due to the poor box office performance of Godzilla vs. Biollante and also to the unlikely success of the film overseas, the film was cancelled.
2,000 years ago, a giant monster named Bagan protected ancient China. The kaiju rose to destroy threats to the Earth. Because of this, Bagan was simply referred to as a yamato, or living god of the forest. However, as time passed, he grew hostile and began ravaging the forests, striking fear and terror into the inhabitants. Although the ancient people of China were able to dispell the creature, ancient Chinese legends that were passed down throughout the ages claimed that the "primordial dragon" would rise to defend the planet again.
In the present day of 1990, the glacier containing Bagan melts away from overexposure to rising global temperatures, and the monster sets its sights on destroying humanity, now Earth's greatest threat. However, standing in Bagan's way is Mothra, a giant guardian moth left behind by an ancient civilization to defend the Earth. Mothra confronts Bagan in Singapore, but is soon overwhelmed by the demonic beast. Some distance away, in Borneo, a gigantic egg begins to stir, and it soon hatches an infant Mothra, which crawls out and arrives to help its mother. The three monsters then engage in a long battle in Bangkok. Sadly, despite the combined efforts of both Mothras and the poison powder, the adult Mothra is killed by Bagan, leaving the larva with the tides of battle. The larva quickly retreats to a remote island and spins a cocoon.
Back in Bangkok, Bagan sprouts two gigantic wings that allow him to fly about and levels the city. The next morning, the larva comes out of the cocoon as a fully transformed adult Mothra. The two monsters then fight again for the last time in Bangladesh. Bagan, though, has become even more powerful, due to his strong wings. Thankfully, after a long brutal battle, a weakened Bagan is finally defeated by the younger Mothra and humanity is saved. Meanwhile, at the bottom of the ocean, Godzilla awakens, having been disturbed by the battle between Mothra and Bagan.
- Some of the plot for this movie was re-incorporated into Rebirth of Mothra.
- Desghidorah's design choices were very similar to Bagan's, as they both grew wings just before the final battle.