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.