Jack Shindo and Stanley Haggard are members of the first manned expedition to Mars, and on the red planet find a giant slug-like monster, Goudes/Gudis. Shindo is pinned by a rockslide and Haggard tries to escape in their ship but is blown up by Gudis. Suddenly a giant warrior, Ultraman Great, arrives and fights Goudes, but when Ultraman is on the verge of victory Goudes changes into a virus and travels to Earth where it mutates other creatures into monsters and awakens existing ones. Needing a human host to survive on Earth, Ultraman joins with Jack, allowing him to become the mighty alien when all seems lost. He joins UMA in order to help them battle the monsters..