Set in an alternate universe in the year 2007-2010 (2049 in the U.S. dub), giant monsters and conquering aliens start to appear, as was foretold by an apocalyptic prophecy about an uncontrollable chaos over the Earth. Facing the threat, the TPC (Terrestrial Peaceable Consortium) is created along with its branch, GUTS (Global Unlimited Task Squad). Through a holographic message in a capsule found by researchers, the GUTS gets knowledge about a golden pyramid built by an ancient civilization. At the site, three statues of a race of giants who defended early human civilization on Earth about 30,000,000 years ago have been unearthed. GUTS finds the three ancient statues, but two of them are destroyed by the monsters Golza and Melba. The third one gains life from the spiritual energy of officer Daigo, descendant of the ancient race. Daigo and the remaining statue merge into a single being, made of light. Shortly after defeating the two monsters, Daigo is revealed by the hologram of the prophecy that 30 million years in the past, a great evil that not even the giants could stop, destroyed the ancient civilization.
The same evil reappears in the finale of the series, the Ruler of Darkness Gatanothor, and his servants, Gijera and Zoigar. Gatanothor defeats Ultraman Tiga with ease, withstanding the Delcalium Light Stream and a modified version of the Zeperion Ray, both Tiga's finishes, and turns him back into a stone statue, but the light of humanity is able to turn him into Glitter Tiga, giving him the power to defeat Gatanothor and save the Earth. However, Tiga's victory came at a cost. Daigo was no longer able to become Tiga after the Spark Lens disintegrated into dust after his final battle. It is revealed that Tiga, although no longer bound to Daigo, and its energy now remain in the hearts of all those who believe in Tiga, inner-strength and justice. Given the right conditions such as times of despair, the sparks will gather and the Tiga statue will be revitalized.