Fern reveals his ability to change his appearance to look just like Finn. Impressed, Finn joins Fern to a dungeon hidden away in a mountain. While there, Fern traps Finn in one of the rooms and reveals his plan to replace him. Finn finds out that his robotic arm can do a variety of things and drills his way out of the dungeon. Finn and Fern fight out in the field where Finn attempts to reason with Fern, however his robot arm misinterprets the word "finality" with "fatality" and he ends up destroying Fern. Finn returns home traumatized with Jake and BMO comforting him. Meanwhile, a mysterious man (voiced by Fred Melamed) scoops up Fern's still living remains in a bucket.