Hair, I asked my friend what he thought the plunge was in TTT. He seemed to think that it was
Gandalf's fall in Moria! I asked him if he was sure and he said:
"I'm not sure of it, no, but I'm pretty certain." The only other bit of info I got from him today was the the very last "big" event in the movie (but maybe not the final actual scene) is "Aragorn getting Anduril and deciding to go on the Paths of the Dead".