The military starts out by attacking Enjin Reijuu, and after being injured retreats into a cave, from which shortly after the Doragon Reijuu emerges from. The attack continues, now injuring the Doragon Reijuu, who retreats toward the sea. It immediatly dives into the sea, and transforms into Mizu Reijuu. It is at this time that the creature is dubbed Bagan, since it is realized that the three creatures were actually one. Bagan's ability, besides changing shape, was to renew his own structure each time he did it, and there by healing himself. The military sends the Super Beetle (a new underwater mech) to destroy Bagan, its mission is unssecful however when Godzilla arives on the scene to attack Bagan. The battle is fierce, Bagan manages to injure Godzilla, and the Super Beetle is destroyed. Godzilla faints in the sea and Bagan flies off into the sky. A short while later Godzilla attacks a near by Nuclear Power Plant and renews his energy. He finds Bagan again and the battle continues, this time Godzilla destroys one of Bagan's wings while he is in Doragon Reijuu form, and Bagan goes crashing into the ocean and makes his final transformation.

From the chest of Doragon Reijuu, the arms, legs, and face of the Mizu Reijuu begin to form, then the arms, legs, and face of the Enjin Reijuu also take shape on Doragon Reijuu's body. Which reveals Bagan's final form, however once Bagan is in this form he can not change out of it, and as a result can no longer heal himself. Godzilla makes quick work of Bagan's new form, and Bagan dies falling into the sea.

Bagan would return for the screenplay of Mothra vs. Bagan, but would appear much differently then the one in The Return of Godzilla (ver. 1).

Note, no picture of Bagan (1st Generation) exists beyond a concept art drawing for the script. the above concept was created by the Toho Kingdom from the concept drawing, using it as a model and then coloring it in and fleshing it out a bit.