"John, the kind of control you're attempting simply is... it's not possible. If there is one thing the history of evolution has taught us it's that life will not be contained. Life breaks free, it expands to new territories and crashes through barriers, painfully, maybe even dangerously, but, uh... well, there it is." - Ian Malcolm

We start off in Doctor Princess' office. A large egg is right by her desk. Suddenly, a crack is made in the egg, and it hatches into a fierce looking, demonic creature. A while later, in the Fire Kingdom, Flame Princess is waiting for Cinnamon Bun in the throne room. When he doesn't show up, Flame Princess has one of her guards go out and search for him. Just then, Cinnamon Bun bursts in, mangled and mutilated. "IT'S COMING! IT'S COMING!" he yells as he dies near Flame Princess' feet. Meanwhile, in the Ice Kingdom, Marceline, Princess Bubblegum, Finn, Jake, and Ice King are in the middle of a game of croquet, which Finn dismisses as "lame". Just then, Marceline notices an explosion in the distance, and the group checks it out. There, they find Magic Man, injured. When asked who did this to him, he weakly says he doesn't know what it was. Just then, the creature from Doctor Princess' office appears, now 30 feet tall. Marceline recognizes the creature from a folk tale within the Vampire Kingdom. It's name is "The Fudmantor". According to legend, every hundred years, the creature is reborn and seeks to devour any lifeforms it comes across, but is always defeated. Just as Marceline finishes explaining the legend, the Fudmantor attacks the group. Jake shapeshifts into a large ogre and battles the creature. Just as the Fudmantor gains the upper hand, Flame Princess flies in and launches fireballs at the monster, drawing it's attention away from the group, who use the opportunity to escape. Princess Bubblegum asks Marceline how they'll defeat it, but Marceline says it's been defeated in various manners. Meanwhile, Flame Princess has tired out from the conflict, and the Fudmantor grabs her in it's claws and prepares to devour her. Later that night, the group is in the boneyard from "Henchman", thinking up a plan to defeat the creature, and Marceline says she came up with a brilliant plan, and needs Finn to help her out with it. The two fly out of the area just as the creature returns. Ice King challenges the beast, but is quickly devoured by it. Princess Bubblegum and Jake engage the creature, but are easily beaten. Meanwhile, in a cave, Finn and Marceline come across an ice wall, in which Rexy the Tyrannosaurus is frozen. Marceline asks Finn to plug a nearby heater into a nearby generator, which he does as Marceline lights a flare. The heater thaws out the ice, releasing Rexy. Marceline then asks Finn to get on her back as she guides Rexy to the Fudmantor using the flare. Once back at the boneyard, Marceline throws the flare at the Fudmantor, and Rexy crashes through the dragon skeleton and roars in challenge. Princess Bubblegum and Jake leave the area with Finn and Marceline as the two giants do battle. Even though Rexy has the advantage for a while, she is eventually brought to the ground by the Fudmantor's strength. Before it can deliver the final blow, however, Flame Princess, who survived her previous struggle with the Fudmantor, calls out to the beast, and launches a barrage of fireballs at it, buying Rexy enough time to return to her feet and continue her assault. Eventually, the Fudmantor is thrown onto the broken rib of the dragon, and is impaled, and Rexy rips off the creature's head, slaying it once and for all. After the fight, Rexy and Flame Princess look at eachother, but Rexy decides to spare Flame Princess, thanking her for her help before walking away. Then, Finn and Flame Princess look at each other for a moment, and Flame Princess asks "You up for troll slaying next week?", and Finn and Flame Princess hug eachother as the sun rises. In the mountains, Rexy makes her way to the top and roars in triumph while surveying the land of Ooo.