Ultraman Gaia was the 13th show in the Ultra Series. Created by Chiaki J. Konaka and produced by Tsuburaya Productions and Mainichi Broadcasting System (MBS TV), Ultraman Gaia was aired from September 5, 1998 until August 28, 1999, with a total of 51 episodes.
In this instalment, the story takes place in the year 2000. CRISIS, a supercomputer developed the Alchemy Stars (a global network of young geniuses born during the 1980s), predicts around 1996–1997 that Earth and humanity would be annihilated by something known as the “Entity that Brings Radical Destruction”. Technology developed by the Alchemy Stars is used to form an international defence organization known as Globalcentric Universal Alliance against the Radical Destruction (G.U.A.R.D.). This is done secretly, so as to avoid worldwide panic. The eXpanded Interceptive Guards (X.I.G.) is the combat wing of G.U.A.R.D, operating in a floating sky fortress known as the Aerial Base.
20-year-old Gamu Takayama, a scientist and member of the Alchemy Stars briefly encounters Gaia during some virtual reality experiments he performs with the secret purpose of discovering the will of the Earth, later merging with him in order to fight various monsters that threaten the safety of the Earth. During his battles, he encounters Ultraman Agul, whose human form is Hiroya Fujimya, a former Alchemy Stars member. Both clash because of their ideals regarding the protection of the Earth but eventually resolve their differences to battle their common enemy.
Ultraman Gaia takes place in yet another universe, one that is totally different from the world of the Shōwa era Ultramen(Ultraman to Ultraman 80), as well as the world of Ultraman Tiga and Ultraman Dyna. In a departure from most of the other series, there are two Ultramen as the main characters. They have conflicting philosophies in regards to defending the Earth from a mysterious, malevolent and extremely intelligent cosmic entity.
The first half of the series explores the growing tension between them, and their eventual clash. The second half has them reconcile their differences so they can defeat their common enemy.
The show is also different from past Ultra Series in that the support team, XIG, has more personnel than the defence teams in previous series.
In this series, Ultraman Gaia is the lead, but Ultraman Agul makes many appearances. They are often at odds with each other in the beginning of the series. Their philosophies are as different as their skin colours. Unlike Gaia, Agul is completely blue. Gamu Takayama, the human host of Ultraman Gaia, believes that Gaia is here to save Earth and humanity. On the other hand, Fujimiya Hiroya, the human host of Ultraman Agul, believes Agul to be Earth’s natural defence mechanism. He is at first more interested in protecting just the planet itself, even at the expense of humanity (hence the occasional clashes with Gaia). But this was discontinued after Gaia and Agul would work together as one to face more powerful monsters.