When the monster Gudis destroyed Jack Shindo's spaceship and trapped him on Mars, an alien giant from Nebula M78 shows up and defeats Gudis. In order to save Jack Shindo's life, this giant merges with him and gives him a device that allows him to turn into this giant whenever he needs.
After the merge, Jack Shindo can mind communicating with this giant. He knows that this giant is Ultraman Great, a warrior from Nebula M78 just like the Ultraman who used to come to earth and fight monsters before. Ultraman Great's mission is to destroy Gudis, the monster who polluted the universe and fought with Ultramans for millions of years.
After defeating Gudis, several monsters on earth, and the space monster Kilazee, Ultraman Great used up most of his energy to sustain himself on earth, so he left Jack Shindo's body and flew back to his home planet.