(An original story, so don't overthink it. The protagonist is practically invincible, there is no female lead, and his new forms will be explained.)
Li Ze transmigrates into the Ultraman universe, taking the place of a member of the Seven lineage who was captured and modified by aliens. He thinks he is going to die on the operating table, but the Space Garrison arrives just in time and rescues him to the Land of Light. He then takes part in the Space Garrison's entrance exam.
On his very first day, though, he is assigned to a new department. Apparently, the King of Ultra personally ordered Li Ze to capture alien fugitives who have escaped to other universes. According to the King of Ultra, some aliens have somehow found their way into other universes, despite possessing technology far beyond anything they should be capable of. So, are you ready to travel between universes?!