so Finn went on an adventure to find his mom and the last of the humans which worked out pretty great, Susan regained all her memories and she ran off with a childhood friend
Then when Finn got back to Ooo he found it messed up by Patience St Pim who had basically divided the land between the 4 elemental forces. Magic Betty told Finn she knew how to fix things so Finn went around gathering the magic royal jewels for her, however Betty was lying and it was all a part of an insane plan to go back in time and stop Simon from putting on the crown so that he would never become Ice King and he and normal Betty would live happily ever after, luckily this failed.
Meanwhile Finn was able to figure out that LSP was the key to fixing Ooo and with her help he turns everyone back to normal. It ends on a cliff hanger where Finn and Jake reunite but being returned to his true form, Jake now looks like some wacked out alien demon