After an accident sends me to the Ultraman world, I become a Reionics or gain the ability to transform into an Ultraman. Of course, which one is random. (I may get the other one later, too.)
I became a Reionics and got my first monster, Golza.
Both the Monster Battlenizer and the Ultraman transformation device have some ability to travel through time and space, though they need time to recharge.
I have a long life (my constitution is simply better and my lifespan longer, but that doesn't mean I can't die). Still, I'm not the only one, nor am I especially unique. People like me will appear along the way; I can team up with them, but I should also be wary of others.
During this time, I learn there may be a way back to reality and try to return...
