Good question. The answer is "not much."
They make this big deal about soem sort of secret underground, but there's nothing there. You (the GM) sort of make things up. Maybe there's a major conspiracy. Maybe not. At the beginning, the game relies on your characters not knowing anything about the underground.
By the time you're powerful ("high level"), a thematic meta-plot starts to arise. People are trying to join the Invisible Clergy, essentially becoming such great avatars that they become the archetype itself. Maybe you want to do that too. Maybe you want to stop people from doing it. The problem is beginning characters know nothing of this so there's nothing innate holding them together for it. It's easily worked around by just using a dual plot campaign. Part 1: find out about your powers and the underground occult. Part 2: rise in power in the occult (or try to kill the occult, there's a single organization described that does that, and it's the most interesting: no one in the group has magick, and they're all Catholic priests).
Either way it still leaves a gaping hole. The adepts and the avatars aren't supposed to understand each others' theories. They're not antagonistic, they just think the other philosophy has it wrong and won't go anywhere. But how do you reconcile that with adepts suddenly trying to get somone in or out of the Invisibile Clergy? Uh... you don't. Not without rewriting the whole relationship adepts have with avatars.
Again, I liked the ritual magic ideas best. I can see writing a game where the success of the magic comes from players defining how they're gonna do the spell, and having the effect reduced each time they repeat a casting the same way.
