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 terrorized ancient China. The kaiju, known as Bagan, rose to destroy threats to the Earth. Although the ancient people of China were able to defeat the creature, legend passed down through the ages claimed that the "primordial dragon" would rise again to defend the planet again. In the present day of 1990, the glacier containing Bagan melts from rising global temperatures, and the monster sets its sights of eliminating humanity, now Earth's greatest threat. However, standing in Bagan's way is Mothra, a giant insect monster left behind by an ancient civilization to defend humanity. Mothra confronts Bagan in Singapore, but is soon overwhelmed. Some distance away, in Borneo, a gigantic egg begins to stir, and it soon hatches an infant Mothra, which crawls to help its mother. The three monsters then engage in a series of battles that span the globe. Unfortunately, as the monsters fight in Bangkok, the adult Mothra is killed by Bagan. The larva quickly retreats and spins a cocoon, transforming into an adult Mothra. The two monsters then meet again, this time in Bangladesh, and begin the final battle. Bagan, though, has become even more powerful, and has now grown wings that allow the beast to fly. Fortunately, after a long battle, Bagan is defeated by the younger Mothra and humanity is saved.
- Some of the plot for this movie was re-incorporated into Rebirth of Mothra.
- Desghidorah had a similar plot to Bagan, as he grew wings near the final battle of Rebirth of Mothra.