Alright, I will try to explain it, as it's storyline is kinda broken up in 2 movies: Ultraman Zero (the son of Ultra 7) fights against the ultimate evil Ultraman, Belial, since he somehow survived at the end of Mega Monster Battle. He is now in another universe (yes they use the multi-verse theory) and now calls himself Kaiser Belial. Zero is sent to this other universe to stop him, and he meets the heroes of that universe who help him on his quest.
I do suggest that you start with Mega Monster Battle: Ultra Galaxy Legends The Movie if you want to understand who Zero and Belial are. Then watch the Ultraman Zero Movie (link below).
Here is the link:
.html]Ultraman Zero Movie