You have to do some side stuff later on to get the last bit of story. A lot of people ended up missing the actual end to the game because of this. I would describe what missions they are but they're pretty spoilery. They involve playing a certain "game" with someone.
I support it only because I think invasion is one of the games most interesting mechanics and that it's a shame most opt out of it after just one bad experience. They just need to find a way to keep players of equal strength and gear level grouped together.