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). 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.
Thousands of years ago, a giant monster known as 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. 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 of destroying humanity, now Earth's greatest threat. However, standing in Bagan's way is Mothra, a giant guardian moth left behind 12,000 years ago by an ancient civilization to defend humanity. Mothra confronts Bagan in Singapore, but is soon overwhelmed by her age. Some distance away, in Borneo, a gigantic egg begins to stir, and it soon hatches an infant Mothra, which crawls out and leaves to go help its mother. The three monsters then engage in a long battle in Bangkok. The adult Mothra is killed by Bagan when the fight comes to an end, 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. The next morning, the larva comes out of the cocoon as a fully transformed adult Mothra. The two monsters then fight again in Bangladesh for the last time. Bagan, though, has become even more powerful, due to his strong wings. When Bagan gains the upper hand, Mothra figures out the creature's weakness: Bagan is too eager for vengeance. Mothra uses this against the yamato, using effective speed tactics and weakening him. Thankfully, after a long brutal battle, a tired and weakened Bagan is finally defeated by the younger vengeful Mothra and humanity is saved. As the sun sets, Mothra flies back to Borneo.
- 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.