|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 #35 is the thirty-fifth issue of the Adventure Time comic book. This issue features a back-up story titled "Mystery Plop" (Part 2), which was written and illustrated by Kat Leyh.
"Finn has made it to Mnemonoid at last! It's time for this adventure to end and for Finn to finally get his memory back. It might seem like an easy end, but it looks like there is still one more trick up Mnemonoid's sleeve."
An elderly Finn realizes that the Mnemonoid ate his memories again, but is pretty sure he knows how to beat him. He then does the "It's Adventure Time!" bit by himself since he's alone. He packs up his stuff and surmises that he is in the Ice Kingdom, but sees that it's actually the Candy Kingdom.
Finn enters the castle and finds Bubblegum sitting alone in the dark. He tells her that he lost his memories and that he has a way to stop the Mnemonoid for good, but she tells him to leave. She goes on to explain that whatever plan he thought of was attempted many times. Nothing worked and much was lost. She starts pushing him out the door, but Finn asks her to listen. He says they just need to learn how to beat the Mnemonoid. Bubblegum isn't amused, so Finn whispers his plan in her ear, and she is renewed with hope. They head to the lab, though Bubblegum still seems saddened over Marceline's death.
Weeks later, Finn and Bubblegum find the Mnemonoid, using his telekinesis to make a shelter in the snow. Finn says they came to talk and not fight, but the Mnemonoid is uninterested and attacks them. As they fight, Finn asks why the Mnemonoid eats memories, who replies that he does it for sustenance. He says that what he actually eats are stories and that memories are an easy way to get them. After hearing this explanation, Bubblegum sends a swarm of robots to attack the Mnemonoid, who easily stops them. This is actually a distraction for Finn to use his concealed time machine arm. Bubblegum explains that Finn and Jake had an adventure with her time machine which self-destructed. With the plans for it long gone, Finn used her brain projector to show them to her. They then travel back to 60 years ago.
Back at the events of Issue 31, Finn and Jake are talking about driving when Future Finn and Future Bubblegum arrive. The two Finns, excited at meeting each other, do the "Adventure Time!" bit together. Future Bubblegum introduces themselves to Jake and asks BMO to call Lumpy Space Princess and Marceline. Ice King bursts in (upset about his birthday being forgotten) but stops in confusion upon seeing the future pair. Future Finn asks him to sit quietly and wishes him a happy belated birthday. LSP then comes in through the hole Ice King made.
The Mnemonoid shows up to steal Finn's memories, but gets slightly confused at seeing Future Finn. Future Finn then asks what Melissa said to Corey last night, making LSP go off on an endless story involving several people, which the Mnemonoid finds perfect. He apologizes for trying to take Finn's memories, saying LSP's stories are better. Future Bubblegum reminds him to talk with LSP when he's hungry. Princess Bubblegum and Marceline then crash into the room in their cars, and Future Bubblegum hugs them out of joy. With the timeline changed, the future pair starts to fade out of existence. Future Finn tells his past self how to avoid the bad events he went through, and Future Bubblegum whispers something to her past self and Marceline which stuns both of them.
The future pair are gone and the Mnemonoid and LSP are still talking. Jake asks Finn what Future Finn told him. Finn says he was told stuff that can easily be avoided and/or fixed, but was also told to not worry about things changing. What really matters is what happens right now. Bubblegum and Marceline ask them if they want to go driving, to which Finn says yes. The four of them, along with BMO and Ice King, go to drive cars off the ramp. The Mnemonoid and LSP are still in the kitchen and getting along, then Finn crashes in with a car, saying that he doesn't know what he's doing.
Later that night, the Mnemonoid leaves the tree fort (which now has several cars rammed into it). He notices the reader and tells them that the story has ended, but still tries to tell them parts from LSP's story.
Mystery Plop (Part Two)
Back on the giant red plop, Princess Bubblegum thinks she misinterpreted the readings. Marceline arrives with the three vampires, who now support her. Bubblegum begins to tell Marceline that the plop isn't what she thought it was, but one of the vampires bites into it, causing it to pop. It turns out that the plop was actually a balloon that a little man was blowing up for his set. Bugglegum tells him that he made it too big, and he admits that he zoned out while doing it. Bubblegum shoos him away, and the vampires express having a good time (even though one is quite fazed from the balloon popping). As they leave with the balloon's remains, Marceline asks Bubblegum how she didn't figure out it was a balloon. Bubblegum shrugs.