Dipper feels self-conscious that Mabel is taller than him and is becoming the alpha twin, so he seeks out a magic way to grow himself the same height as her. Meanwhile, Li'l Gideon takes advantage of Dipper's and Mabel's height discrepancies.
The episode starts with Gideon inside his room, planning revenge on the Pines family. After considering different options, such as summoning demon caterpillars, he decides that gaining control of the Mystery Shack is his best choice. Gideon's lawyer appears at the front door of the Mystery Shack. Stan mistakes him for a tax collector and tries to hide his money elsewhere. The lawyer then announces that Stan has won ten million dollars, with the stipulation that he signs a contract before getting the money, which Stan seemingly agrees to. The huge ten million dollar check rips apart as Gideon reveals himself. He says that he tricked Stan into giving him the deed to the Mystery Shack, but instead of signing his real name, Stan had actually written "SUCK A LEMON LITTLE MAN," foiling Gideon's plan. Gideon swears that he takes revenge on the Pines family, though not intimidatingly due to his height.
Later, Dipper and Mabel play a game of chess when Soos asks Mabel to get a brain in a jar for him from a shelf above. Dipper asks why Soos chose Mabel over him, and Soos replies that Mabel is taller than Dipper. Dipper denies this statement, so Soos measures them to make sure. Mabel is revealed to be taller than Dipper by one millimeter. Mabel and Stan start making fun of Dipper's height, and he storms off to his bedroom. Looking through Journal 3, Dipper realizes that there are height-altering crystals in the forest, and he goes to investigate. He comes across several crystals that cast blue and pink light. The pink light makes things smaller and the blue light makes things bigger. He takes a crystal and fastens it to his flashlight to make himself a millimeter taller than before.
Mabel still believes she'll be taller in the end, so Dipper zaps himself again. Mabel then thinks Dipper is using something magical, and questions whether Dipper has a wizard in his closet. Dipper finally gives in and tells her about the flashlight, which Mabel instantly wants. Gideon arrives at the door of the Mystery Shack with Cursed Egyptian super termites, but Stan stops him from letting them devour the Shack as Gideon had planned by knocking the jar over, and the termites attack Gideon instead. After Mabel and Dipper fight for control of the magic flashlight, it falls into Gideon's hands, and, after Mabel unwittingly tells him what it does. he shrinks the two and puts them in the empty jar, which he takes back to his house.
In Gideon's room, Gideon places Mabel inside a bag of Gummy Koalas, where she eagerly takes a bite out of one. Gideon then interrogates Dipper about if he read about the crystal somewhere. Dipper then blows an air horn in Gideon's ear. Gideon nearly squashes Dipper in anger, but decides to use them. Soos and Stan finish setting up the Maze of Mirrors, a new attraction, when the phone rings. Stan goes to answer it, leaving his fez behind, which Soos puts on himself. On the other end of the phone is Gideon, who claims that great harm will befall Dipper and Mabel if the deed to the Mystery Shack is not given to him. Stan refuses, and Gideon decides to go to the Shack, armed with the magic flashlight, to shrink Stan and take the Shack for himself.
Dipper and Mabel escape using hair from a hair brush (fashioned into a rope), and ride a balloon to the Mystery Shack. Once there, Mabel distracts Gideon by messing up his hair with a gummy koala, and prepares to enlarge Dipper, however the duo argue about their height difference, and Gideon captures them. He then shrinks what he thinks is Stan; in reality it was just Soos wearing his fez hat.
After Soos inadvertently reveals Stan's location, Gideon sticks the jar containing the trio into his shirt pocket and heads to the Maze of Mirrors. Dipper asks Mabel why she was making fun of him, and she reveals that Dipper beats Mabel at pretty much everything, and she wanted to win at something for once. They quickly make up and escape the pocket. The trio climb up Gideon's jacket and along his neck, but Soos is knocked off. Gideon decides to break the mirrors to find Stan, but he appears in front of Gideon, demanding that he pays $500 for each mirror he broke. Dipper gets an idea, and he and Mabel tunnel under Gideon's shirt to his armpit, tickling him so much that he can't attack Stan, who simply rolls him out of the Mystery Shack. Mabel and Dipper return to normal size using the magic flashlight. Mabel stomps on the crystal to destroy it, the broken crystals spell out HELP SOOS. Dipper and Mabel realizing Soos is still small, they decide to glue the crystal back together to change him back to his regular height.Gideon is pacing around his house, and his dad says he will get his revenge one day. Gideon angrily tells him that it's not just about revenge; he wants the Shack, the physical building, because there is a secret hidden inside of it. He then laughs maniacally while Bud feeds him ice cream. In the credits scene, another person arrives at the door, telling Stan he's won $10 million. Stan simply closes the door in disgust, and the money is offered to Old Man McGucket, who then eats the huge check.
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- Dipper is a millimeter shorter than Mabel.
- Stan isn't familiar with texting.
- Old Man McGucket's full name is Fiddleford H. McGucket.
- Bud Gleeful is a used car salesman.
- Gideon is extremely ticklish, and hates being tickled.
- The model of the Mystery Shack made of Popsicle sticks and the dolls of Mabel, Stan, and Dipper that Gideon creates in "The Hand That Rocks the Mabel" reappear.
- The painting that Grunkle Stan stole from the Gleeful household in "The Hand That Rocks the Mabel" is still missing, leaving a mark on the wall.
- Gideon mentions the shack housing a powerful secret, a reference to his belief that Journal 1 is buried on the Mystery Shack property in "Gideon Rises."
- Viewership: This episode was watched by 2.599 million viewers on its premier night.
- Gideon's school ID number is 618, a number that commonly appears in the series.
- The "Expecto" spell is an allusion to the Expecto Patronum spell of the Harry Potter franchise.
- During the end credits, there was a cryptogram that read "GSV RMERHRYOV DRAZIW RH DZGXSRMT." Once decoded, it reads "THE INVISIBLE WIZARD IS WATCHING."
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