Well, a few months with no activity...That has to be remedied!
So, Finn starts having weird dreams, migraine headaches, memory loss and a number of odd injuries he can't remember getting. Worried about the hero, PB does every test she can but can't find anything Physically wrong with him, until she keeps him under observation. She notices Finn acting odd, muttering about madness, and is about to check when he shifts, suddenly attacking himself, arguing with himself. Jake and Marceline manage to restrain him, but it's only later under Hypnosis that PB is able to find the cause.
Finn's mind is being torn apart by two other, opposing psyches, one a farm boy, broken and seeking to cover the world in ice, the other an aged hero who tries to stop him, but wants to return home to his family.
Now Finn's friend have to find a way to save Finn while trying to figure out where to other Finn's with whole lifetimes of memories came from.