|The following information is from the Adventure Time comic book series. It may not be part of the same canon or continuity as the animated series.|
Adventure Time #31 is the thirty-first issue of the Adventure Time comic book. This issue features a back-up story titled "Mystery Plop" (Part 1), which was written and illustrated by Kat Leyh.
"Finn is on an impossible mission to break Magic Man’s curse so that he can finally live his life…and remember it! It’s going to take a lot of pal time and a lot of help from the best of friends but this definitely seems like an adventure that Finn and Jake can take—if only Finn could remember."
Finn is shocked by his aging after five years and Jake tries to console him, also showing Finn's arsenal of prosthetic arms. Finn still feels something is wrong, so Jake shows him to Bubblegum, Marceline and BMO outside. When Bugglegum hears that Finn can't remember the last five years, she and Marceline make a comic to explain. The Mnemonoid apparently tried to eat Finn's head, so the others beat him up and threw him in Castle Lemongrab. Bubblegum later researched and found that the Mnemonoid eats memories. She ran tests on Finn and thought he was fine, but the effect was actually time-delayed. Everyone decides to give Finn a tour of Ooo and also get his memories back. Using BMO's giant robot legs, they show him the things that changed: the Candy Kingdom is now the Candy/Vampire Kingdom, the Ice Kingdom is more metallic, the Fire, Water, Air, and Sand Kingdoms are fighting, Lumpy Space is Lumpier, and the Breakfast Kingdom is now the Brunch Kingdom. It also turns out that Finn has dated a lot of people. Suddenly, the Mnemonoid appears and restrains everyone with telekinetic fields. He tells Finn his memories were delicious, recalling his favorite one where Finn defeats a giant slug monster. He wants more, but Jake frees himself with his stretchy powers and punches him. Marceline attacks, but the Mnemonoid restrains her and everyone else again. He then pulls out the Lich minion Jake ate (which never got digested) back in the Lich's dungeon, which attacks Jake. With no one able to do anything, the Mnemonoid starts to eat Finn's memories again, but they claim they will cure him. Finn then wakes up to find he is even older and married to Flame Princess.
Mystery Plop (Part One)
Bubblegum is analyzing a massive red blob and doesn't like the readings. Marceline shows up after getting her message. Bubblegum says that the plop just appeared out of nowhere, is steadily growing, and could devastate the Candy Kingdom. However, she thinks that removing the red will stop it. Marceline knows she can't do it alone, but Bubblegum says to get other vampires to help, as Marceline is their queen. Marceline goes to a cave where three vampires are watching TV. She asks them to help, but they refuse to help the Candy Kingdom, calling her lame. Angered, Marceline turns into a bat monster and says she won't offer them free food again, to which one of the vampires says she didn't mention free food.