Ultraman Gaia V1(left) : Gamu Takayama(right)

Ultraman Gaia begins another trend of unique Ultra-heroes. In a different "world" from Ultraman Tiga  and Ultraman Dyna .Gaia is one of two Earth-based Ultraman. Gamu Takayama, one of a group of boy geniuses known as the Alchemy Stars, discovers a realm where a giant Ultraman fights a huge serpent. Gamu is quickly drawn back to the real world. Later that day, a large wormhole opens up near Earth, depositing the huge rock-like carrier of the Cosmic Organism Vanguard. Gamu cheers when he sees Team Lightning, part of the eXpanded Interceptive Guardians (XIG) branch of the Geocentric Universal Alliance against the Radical Destruction (GUARD), take on the beast. His hopes fail when the planes are decimated by the COV. Suddenly time stops around him, and the ground opens up beneath him, carrying him through a tunnel of light like the one he experienced earlier. He again meets the red-and-silver giant, which Gamu immediately identifies as Ultraman (curiously enough). Gamu begs Ultraman to come with him and defeat the monster. When Gamu returns to Earth, he has adopted the form of Ultraman Gaia. After managing to defeat the monster, Gamu joins XIG as a technical advisor.

XIG is composed of several teams, including 3 fighter teams and 3 "expert teams" or teams of specialists. The 3 fighter teams are Team Lightning (the Ace Pilots team), Team Falcon (the Veteran Pilots team), and Team Crow (the Lady Pilots team); the 3 expert teams are Team Hercules (Ground Duty team), Team Seagull (Rescue), and Team Marlin (Ocean). The Peace Carry is a larger command ship that can carry one of the teams' ships as needed. XIG's headquarters is the mobile Aerial Base, a large airship that hovers above the Earth, using technology originated by Gamu himself.

Gamu's teammates include Commander Ishimuro, Staff Officer Chiba Tatsumi, Chief Tsutsumi Seiichiroh, Sasaki "Akko" Atsuko, and Georgie Leland (played by American-born 16-year-old Maria Theresa Gow, an amazing lady who can speak both English and Japanese fluently).

Gamu transforms into Gaia through the Esplender. Curiously, Gamu builds the Esplender himself, modeling its design after the ColorTimer on Gaia's chest. Before he builds the Esplender, Gamu traps the "essence" of Gaia in a glass vial from his lab, and uses it for his first transformation. (Again, Gamu's actions indicate that this world, though not connected officially to that of Tiga and Dyna, does know of the Ultra-warriors in its own way.)

Gaia's main attacks include the Photon Edge (a "blade of light" fired from his head), and the Quantum Stream (a typical Ultra-style beam, though fired by crossing his hands-style, then rotating them around before firing). Gaia normally appears at an Ultra-standard 50 meters tall, but he can shrink to "micro size." Gamu appears to feel every bit of pain and punishment inflicted on Gaia; he often staggers away from the scene, feeling bruised and battered, after changing back.

One of Gamu's former associates in the Alchemy Stars is Hiroya Fujimiya, who becomes  also of his own will. While Agul has been a help to Gaia, more often than not they've been rivals, even enemies. Fujimiya reveals that many of the monsters sent through the wormholes actually come from the Entity of Root Destruction, which his super-computer, CRISIS, has predicted. The computer has also predicted the cause of the Root Destruction: humanity. As Gamu cannot believe nor accept this, this puts the two ex-friends at odds with each other. When Fujimiya manages to summon the gigantic serpent-beast Zoalim, he realizes he no longer deserves the Ultraman abilities, and releases the globe that contains the Agul force to Gamu. Gamu draws it into his Esplender (see below), and transforms into a new and improved version, Gaia V2.

Gaia V2 appears similar to the original Gaia, except for a black stripe between the yellow stripes that go around his back and cross below his ColorTimer. Gaia V2 appears to be slightly stronger & faster than the original version. When Gamu realized that he still did not have full access to Agul's energy, by intense concentration Gaia transforms into Gaia Supreme.

Gaia Supreme is the ultimate form of Gaia. (Gamu now transforms into Gaia V2, who can then switch to Supreme mode.) Gaia Supreme is even stronger and faster than V2. His main attack is an energy beam that starts with a motion similar to his old Photon Edge, then culminates into a prayer-like stance. His hands separate vertically, and the beam emits from his hands, plus the space between.

Even though Gaia is, according to Tsuburaya Productions, not set in the same universe as Tiga and Dyna, the Spring of 1999 introduced a new movie, "Ultraman Tiga, Ultraman Dyna, & Ultraman Gaia." In this movie, a young boy discovers a red globe that grants wishes. In his world, the Ultramen are characters on television; presumably, this is set in the "real world." His biggest hero is Gamu, so he wishes him into his world. Gamu is disoriented at first, but manages to discover that he is actually perceived as a television star in this world. Three older bullies steal the globe from the boy, and use it to wish a monster into existence. Gamu changes into Gaia, and defeats the monster, although he discovers his ColorTimer begins blinking almost immediately, though slowly; this is apparently some strange effect of his crossing over. Later, Gamu himself is hunted by a police force who know nothing of GUARD or XIG, except on their kids' favorite TV show. Gamu fades back to his own world before he is captured. The orb takes possession of the three boys, causing them to wish another monster, actually the combination of 3 monsters the boys sculpt, into existence. Meanwhile, Gamu, realizing what had happened, takes a vehicle of his own design and bridges the dimensional gulf. Seeing the triad monster, he first tries to use the machine in robot mode to battle the monster, but the orb, now growing in evil power, supercharges the beast, which destroys the vehicle. In the nick of time, Gamu changes again into Gaia, but is overcome by the monster, which splits into its 3 components.

The young boy, his best friend, and a mysterious new girl in town manage to snatch the orb away from the boys, breaking the spell it has over them, just as the monster nearly destroys the building on which they stand. The boy bravely works his way down to the orb and makes a wish just as the building explodes. The children suddenly find themselves safe in twin hands of light. The boy has wished up help for Gaia, in the form of Tiga and Dyna. The two newcomers join the battle, splitting up the 3 monsters onto land, sea, and in space. Each manage to destroy their respective monsters almost simultaneously. Gaia, his ColorTimer blinking, changes back to Gamu, and thanks the young boy. The orb has returned to its small, red state. The girl takes it, makes a wish, and simultaneously the orb turns blue, all the devastated buildings are restored, and Tiga & Dyna (who never return to their human form, by the way) disappear. Then the girl herself fades out just after Gamu does the same; it seems she had come from another world with the orb. Everything returns to normal, until a new girl comes into class: it's the "real world" counterpart to the orb girl!

Soon after this, Fujimiya reveals that he did not perish in the Zoalim's attack, though he lacks the Agul energy. Frustrated, he tries to drown himself, to be rescued by Gamu. When Gamu is captured by a giant robot, Fujimiya realizes that Earth's last hope is about to be stolen away by the wormhole entity. Fujimiya calls out to the water, and the water responds, granting him another chance to be Ultraman Agul, this time Agul V2 (as his appearance changes slightly; see Agul's entry for a picture). Agul rescues Gamu and destroys the monster. Shortly thereafter, Fujimiya discovers that the prediction that humanity was the cause of the Root Destruction was actually placed into the computer by the great Entity of Root Destruction itself, as two more monsters arrive on Earth through a wormhole created by GUARD itself. (The idea was that GUARD would send a pre-emptive strike missile through the wormhole to the planet on the other end, in the hopes of destroying the Entity. The plan failed, but the wormhole allowed the Entity to send two monsters to Earth.) Agul and Gaia finally team up and, together, defeat the monsters. The rivalry has ended, and the Ultra-team has begun.

The series ends with Gaia and Agul fighting a giant swarm of creatures that completely block Earth from the Sun. All transmissions, except for television broadcasts, are blocked. The creatures combine to form one, two, and even 3 great monsters, battling Gaia and Agul almost to exhaustion. Suddenly a gigantic "angel" appears from the wormhole, destroys the great beasts, and recharge Agul and Gaia. This "angel," significantly larger than even Agul and Gaia, turns out to be the Entity of the Root Destruction itself, and sets about defeating the two Ultraman. Their ColorTimers gone dark, the two collapse in a pile, their eyes even going dark. The angel then extracts the essences of Gaia and Agul into herself, causing the giant Ultramen to shrink down to Fujimiya and Gamu. (While XIG had become aware of their secrets a few episodes previously, now the whole world knows.) The two are not dead, though, and are rescued by their friends. Meanwhile, various other Earth monsters appear, trying to fight off the alien invasion, to no avail. Catherine Bach, one of the prettier members of the Alchemy Stars (who also has a "thing" for Gamu), realizes that the Earth monsters have power that could be made useful. Using specially-equipped XIG fighters, the Stars draw off blasts from the monsters (who seem to have some idea what's going on) and transmit the energy to the spot where Gamu and Fujimiya stand. The light re-energizes them, allowing them to become Ultramen one more time. After vaporizing all the critters covering the Earth, they attack the angel with full force, stunning her. She transforms into a horrible monster, and charges the Ultramen. Realizing that their combined energy had summoned Zoalim in the first place, the two fire their beams at a spot between them, creating a mega-beam that shreads the Entity. Earth is finally safe from the Root Destruction, and Gamu & Fujimiya go off on well-earned rests. (The series doesn't state whether they still have the ability to change to Ultramen, should the need arise.)

Even with the first episode sequence indicating that Gaia was a separate entity, later episodes, plus background information provided by Tsuburaya Pro., indicates that Gaia and Agul, like Tiga and Dyna before them, are "human Ultra-warriors," Ultramen who are purely human, rather than extra-terrestrial in origin.