The Mystery Dungeon is the main location of the episode of the same name. Ice King kidnaps Lemongrab, Tree Trunks, Shelby, and Neptr and brings them here to help him reach the center of the dungeon, at which the Ancient Sleeping Magi of Life Giving resides.
On the walls of the first room in the dungeon are the words:
- Awake, avast, hold tight your buns, if buns you do hold dear, for time has come to wake and run and not give way to fear. Beyond this room are a thousand rooms, outside of them still more. Behind each door, on every floor, danger, danger, ever more. Stranger, hark! The traps galore may kill you whilst you do explore. To free yourself from this tomb, and nevermore re-enter, find the room amidst the rooms that lies exactly center.
After the five finish reading the words, the walls glow red and reveal a map. The walls then close in on them, but they escape through a trapdoor to another room, in which are baking supplies. Tree Trunks bakes an apple pie, which lures a large rat out of a hole in the floor. They then proceed through it to a steamy room with a Monster Fish swimming in a large pool.
Once they kill the fish, they enter a room with a large tentacled monster, who picks up Lemongrab and squeezes the juice out of him. The juice causes the monster to explode, and they enter a room with a large spiral staircase. At the bottom is a door "secured with an esoteric encryption system." Neptr throws a pie at it, causing it to short-circuit and open the way to the room with the Magi.
Ice King throws his Fionna and Cake Omnibus into the Magi's flames, hoping to bring his fictional characters to life. Instead, the book itself becomes alive and starts farting on Ice King. The Magi then exits through the ceiling of the dungeon to the Grass Lands, and Lemongrab, Tree Trunks, Shelby, Neptr, Ice King, and his newborn book follow suit.
- The hole that Magic Man emerges from in "All the Little People" bears a striking resemblance to the hole that Ice King and company use to exit the dungeon in "Mystery Dungeon." It is a possibility that Magic Man used the Magi's flames to bring the Little People to life.