Ultraman Gaia is the titular main protagonist of Ultraman Gaia. He is third Ultra being of Tsuburaya Productions' First Heisei Wave, created by after the 15-year long hiatus of the Ultraman franchise, and the successor of Ultraman Tiga and Ultraman Dyna. His first primary form was Version 1, and he is incapable of form-changing like the previous two Heisei Ultramen. Later, he gained both Version 2 and Supreme Version from Ultraman Agul's light.
Gamu is a 20-year old member of the Alchemy Stars who invented much of the technology used by XIG (including the repulsor lifts that allow Aerial Base and the XIG fighters to defy gravity). During some virtual reality experiments (performed with the secret purpose of discovering the will of the Earth,) he briefly encounters the mysterious glowing red giant, Ultraman Gaia, at the end of a tunnel of light.
When the KHST begins its first wave of attacks on Earth, XIG is too new to be effective. Time stops and he falls through another light tunnel, where he encounters Gaia again. Because Gamu is so passionately concerned with the fate of Earth and humanity, Gaia's light becomes a part of him, and he is thus able to transform into Ultraman Gaia when necessary. The first monster, Cosmic Organism Vanguard, is destroyed. Subsequently, more KHST-generated monsters attack, and Earth monsters are also mysteriously roused, so Gaia is kept pretty busy. Gamu joins XIG as a technical advisor, and later creates the Esplender to store the light that is the essence of Gaia.
At XIG, there are multiple three-person teams with specialized roles. In addition, there are the three commanders and three communications women (Akko and Georgie, plus Ayaka, a support recruit). XIG has more women than some Ultra support teams have both men AND women.
The show also features many non-Japanese actors who speak Japanese fluently. The one most often shown is Georgie Leland, the communcations officer (played by Maria Theresa Gow). Catherine Ryan (Cat) is an American woman who likes weapons and dresses like Sarah Connor and in one episode goes on an impromptu date with Gamu, much to the chagrin of Akko. There's also Michael Simmons, the only visible black member of XIG who is a part of Team Seagull, as well as a few other Alchemy Star members.
Finally, there is the mysterious Hiroya Fujimiya, the dark, angsty youth who transforms into Ultraman Agul. He is responsible for developing the CRISIS computer array and its dire predictions. After many recalculations, what he learns is that the only way to save the Earth from KHST is to eliminate humans from the equation. He quits the Alchemy Stars and goes off by himself and Professor Kyohko Inamori to find the will of the Earth, only to discover Agul.
Agul and Gaia fight against KHST and occasionally help each other out, although Agul makes it clear their alliances are based on his duty to the Earth and his own ambitions, not out of friendship or gratitude. However, as the series progresses, Fujimiya grows more agitated and reckless, and has less and less regard for his fellow humans in his desperate mission to save the Earth. He pushes himself to the point of endangering his own health, and becomes feverish and irresponsible. When bullied in a bar, he declares that humans need to be purged from Earth. But as Reiko the TV reporter points out, how can he take care of the Earth when he can't even take care of himself?
Gaia and Agul have a major clash in Episode 25 and both are badly injured. Incredible energy is released during the fight, which opens up a giant wormhole. An immense serpent named Zoalim partially emerges to invade Earth. It turns out that Zoalim was anticipating this clash, and was able to influence CRISIS's computations. Gamu explains to Fujimiya that Crisis was sabotaged all along, and its analysis is fundamentally flawed. Fujimiya realizes that everything he believes in is based on this flaw, and becomes dejected, losing faith in himself. He gives the injured Gamu the light that is the essence of Agul so that he can use it to defeat Zoalim. Fujimiya disappears, but reappears later in the series, mostly due to fans IRL demanding his return.
Gamu accepted Agul's light into his Esplender, and afterwards could transform into GaianVersion 2; a hybrid of both Gaia and Agul. He resembles the original Gaia, except for a black stripe on his chest plate. Gaia Version 2 can further transform into Gaia Supreme Version, which resemble Gaia V2 with blue colors in his body and a much more muscular physique.
Later in the series, G.U.A.R.D. tries to be more proactive about defeating the monsters that are attacking the Earth. They try to destroy dormant monster Tigris in episode 38 by sending a bomb into the ground (a plan that backfired horribly). Later, in episode 44, they create an artificial wormhole to destroy the home planet of monsters C.O.V. and Pazuzu, also with disastrous consequences.
The K.H.S.T. uses this opportunity to send C.O.V. and a Pazuzu through the wormhole, and they become more powerful as a result. What becomes evident to Gamu is that monsters like Tigris, MIZUNOENORYUH, Shazack, and Izaku, are creatures of the Earth like humans are, and perhaps it is not so fair to eradicate them when their only instinct is to survive. As for creatures sent by the K.H.S.T., they are not inherently evil. When they are sent to Earth their first instinct is simply to survive in an alien environment. Thus, Gamu / Gaia questions his morals and human motives.
There is an environmental spin in episodes involving "Nature Control Machine" monsters. TENKAI is the wind machine of episode 7, ENZAN the heat machine of episode 28, and Shinryoku, the forest machine of episode 46. It is later revealed that they would be used to reconstruct the Earth after humans are exterminated. It is still not clear when they were put on Earth.
Agul eventually returns later in the series, and he and Gaia set their differences aside to fight the common enemy. When Gamu's life is threatened by robotic monster Sigma Zuigul, he pleads with the Earth itself 'to fight one more time' and he is granted the power of Agul again, as Version 2. Version 2 appears to have greater power than Version 1, as he was able to withstand Sigma Zuigul's second attack with little difficulty. Gaia and Agul finally join forces in episode 44 to defeat Super C.O.V. and Super Pazuzu.
The final assault on Earth forces begins with the arrival of BlitzBlotz, a black and white avian monster who has the ability to absorb and fire back Ultra energy weapons with the red object on his chest. He systematically destroys G.U.A.R.D. bases, and roughs up Agul V2 in the process. BlitzBlotz destroys a base that resembles the original Science Patrol headquarters. Tigris II emerges to fight BlitzBlotz and XIG, G.U.A.R.D., monster, and Gaia must fight side by side to defeat Blitz Blotz. Ironically, part of this motley alliance includes the G.U.A.R.D. commander who killed the original Tigris back in Episode 38, as part of an anti-monster vendetta.
Later, giant creature Mokian is sent to Earth in episode 47. He is a living monopole, a monster with enough magnetic power to pull apart the Earth's crust and end civilization. It is revealed by the Death God that the KHST feels humans are a threat to the universe and must be eradicated. This is a fabulous, must-see episode in which the Aerial Base joins the action, and in the process it suffers extreme damage. It is determined that the only way to destroy Mokian is to sacrifice what is left of the Aerial Base by ramming it into the giant monster, and this is done, although miraculously, no one dies in the process.
The last three episodes involve the arrival of an army of bug-like monsters called Dobishi that cover the earth and darken the skies. XIG is in tatters, humanity is losing hope, and the Dobishi just keep coming. They combine to form the giant Kizer Dobishi, and the two Ultramen can't defeat them all.
The KHST finally reveals herself in the form of Zogu, a giant, white angel. She kills the Kizer Dobishis that the Ultramen are fighting, and replenishes Gaia and Agul's power, only to completely overpower the two and make their Life Gauges expire. The two Ultramen turn back into Gamu and Fujimiya, which is telecast by the two KCB cameramen. The battle is broadcast across the entire world. They survive, but can no longer transform into Gaia and Agul.
At this point, all of Earth's monsters start emerging again all around the world to protect Earth from the Dobishis and Kizer Dobishis.
Everyone is brought back and united to participate together in the final battle against Zogu. Gaia, Agul, XIG, the Earth monsters, the Alchemy Stars, the KCB TV crew, and even Gamu's parents show up to encourage him. Gamu hatches a final scheme, in which the Alchemy Stars network is used to assist the last of the XIG fighters (some are flying normal military planes that have been modified by the Alchemy Stars) to channel energy from the Earth monsters to give Gamu and Fujimiya the ability to transform back into Gaia and Agul.
Boosted with this huge supply of energy, Gaia and Agul wipe the Earth clean of the hordes of Dobishi. However, as Gaia finishes off the Dobishi, Zogu reappears. She strikes down Gaia and Agul, but they fiercely counterattack. After being briefly staggered by the counterattack, Zogu morphs from her angel form into a colossal dracotaur (dragon/centaur) monster.
Gaia and Agul then work together as a team and they destroy Zogu with their strongest attack, formed by combining their energy beams. Zogu is vanquished and people come out into the open to cheer their heroes.
Ultraman Gaia: Gaia Once Again
Follows the lives of Gamu and Fujimiya after the events of the original series. In this story, the entire world now knows that Gamu was Ultraman Gaia and Fujimiya was Ultraman Agul. Gamu has resumed his studies at university, while Fujimiya wanders the Earth looking for ways to protect Earth's monsters.
Gamu is reunited with his friends who try to set him up with a girl who is really an Ultra 'groupie.' Fujimiya is framed by a TV report as being the cause of a nuclear disaster, when in truth he was using his cybernetic monster control system to get a monster away from an advancing tank platoon. One of the tanks gets in a lucky shot and causes the monster to detonate, resulting in a nuclear blast. Only the power of Agul keeps Fujimiya alive, but he remains badly wounded. A new menace appears to unite Gaia and Agul in battle once again, and they receive help from unexpected quarters: a pacifist race of undersea creatures unknown to mankind, but coexisting in secrecy for millions of years.