When Grunkle Stan goes on vacation for three days after making a bet with Mabel to see who can make the most money in that time, Mabel is left in charge of the Mystery Shack and decides to run the business in a radically different way compared to her uncle. Instead of looking down on her employees, she encourages them, but at the same time lets them take advantage of her kindness. She also tells Dipper to go into the forest to find a genuine mystical creature to show off as an attraction rather than rely on the tacky fake attractions Grunkle Stan usually exhibits.
Dipper succeeds in finding a monster to display to the tourists thanks to the "3" journal he keeps. The creature is the Gremloblin, a ferocious beast that can show people their worst nightmares by looking into their eyes. This turns out to be a poor choice for an attraction as some unlucky tourists are driven insane by the Gremloblin's gaze. Worse still, Mabel gives the monster a key to let itself out of its cage for a five-minute break. The creature escapes and starts tearing the Mystery Shack apart. As Dipper reads in 3 that water should only be used as a last resort when fighting the creature, as it will make him stronger and scarier, Mabel rushes in and splashes a glass on him, not hearing the full sentence. She is quickly captured by the beast and, just as she looks into his eyes, her brother diverts the creature's gaze to his reflection in a mirror, causing him to see his worst nightmare, wearing glasses like his father. This terrifies the monster and it flies away from the Shack.
The Gremloblin is a seven foot tall humanoid creature with cobbled dark green skin with patches of dirty hair growing on its forearms and small mushrooms growing on its shoulders. It also has glowing red eyes, large fangs and a stocky, muscular build. If exposed to water, it grows larger and develops spikes down its back. It can also sprout a pair of bat-like wings.
The Gremloblin has an assortment of unique abilities, notably the power to make others visualize their worst nightmare, which may result in the victim going insane upon eye contact (this power can also be used on himself if he stares at his reflection). He seems to possess some level of intelligence, evidenced when he reads Mabel's note and then uses a key she left to escape his cage. Additionally, the beast can breathe fire, grows spikes on his back and a pair of wings and becomes more menacing when he makes contact with water. His claws emit neurotoxins.