When he returns to Earth to attend the wedding of his friend in Saipan, Musashi meets a beautiful girl named Shau who claims to come from the bottom of the sea. Just then, Monster Scorpis reappears in the sky above the island and Musashi faces the monster in order to protect the girl. What is the girl's true form? And what is the strange shadowy form that follows the monster Scorpis?

Gamu Takayama is a 20 year old member of an organization called Alchemy Stars. After a monster known as C.O.V. attacked the Earth, Gamu falls through a portal of light and meets the giant: Ultraman Gaia, who gives him his powers. Later, when Gaia was in trouble in episode three, another Ultraman, Ultraman Agul appeared. Can they save the Earth? Who is Agul, is he friend or foe? Later in the series it is seen that Agul, too, is a being of justice, but only protects the Earth, not humanity. The conflict between the two later leads to a duel, opening a wormhole that allows a giant beast to reach Earth. What will happen then?