Musashi is a 10 year old boy with big dreams. He wants to grow up to be an astronaut so he can fly through the stars and meet his hero Ultraman. His mother worries that Musashi daydreams too much, but his police officer father thinks a little bit of daydreaming never hurts anyone.

In Ultraman Cosmos vs. Ultraman Justice: The Final Battle, the two titanic superheroes, Ultraman Cosmos and Ultraman Justice, engage in an epic showdown for justice and the fate of the universe. With the power of their superhuman abilities and amazing technology, they clash in a spectacular battle filled with intense action, alien creatures, and the ultimate display of superpowers.

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?