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.
三千万年前人类已经存在，并拥有高度科技，称为超古代。而迪迦·奥特曼为在超古代造访地球的光之巨人。在经过了人类的光暗战争之后，超古代人类自己选择了灭亡的道路，迪迦·奥特曼等光之巨人尊重人类的选择，身体变成了石像形态封于金字塔内，而灵魂则化成光离开地球。
2007年，怪兽哥尔赞出现，其后一颗陨石堕落地球，陨石中的居然是一个立体投影装置，装置所投影出的人自称是超古代时期的地球警备队队长、人类祭司幽怜，并预言“大地的怪兽”哥尔赞及“天空的怪兽”美尔巴会出现，果然其后马上出现。而预言中说，可以消灭这两隻怪兽的只有迪迦·奥特曼。 GUTS队员大古因继承了超古代人的遗传基因，也被幽怜称为“既是光，也是人”，而成为化身迪迦·奥特曼的人选。
