Li’l Gideon, for some reason, wants to hurt the entire Pines family and sets his sights on acquiring the Mystery Shack for himself.  He tries the old “fake sweepstakes win” to see if Stan will, in his blind greed having achieved his lifelong dream  of having a lot of money, sign his name to just anything, but Stan apparently knows a Gideon scheme when he sees one.
Gideon is enraged. Â The Pines family just goes back to the Ducktective.
But there are more problems. Â Though Dipper can apparently easily beat Mabel at various board games like chess, she is a millimeter taller than he is, and that leads to heavy teasing from Mabel and Stan and Soos. Â Not really on Soos. Â He only did it accidentally. Â Dipper, however, has a book with a number 3 on it, so maybe there’s something in there about fast growing.
Hey! Â There is! Â Deep in the forest is a mysterious crystal that, when exposed to light, gives off purple or blue beams. Â Purple makes things shrink. Â Blue makes them grow. Â Taking a shard home and attaching it to a flashlight means he can now make himself a tiny bit taller. Â And even though Mabel is still claiming to be the alpha twin, Dipper can always try again. Â That just makes Mabel suspicious. Â Sure, the giant chess piece poking through the roof goes unnoticed, but maybe there is a wizard in the closet (there isn’t). Â Sure, it could be an invisible wizard (nope, but Mabel sure thinks so), so why is Dipper taller?
Here’s a better question: Â why does Dipper tell Mabel about his magic flashlight beam?
I mean, she reacts the way anyone in that situation would probably react. Â There’s a fight, the flashlight goes flying after various things are shrunken or grown, and Gideon finds it after another scheme to get the Shack fails miserably. Â He has no problem shrinking the twins, putting them in a jar, and taking them home.
Man, is that his mom? Â She’s so shellshocked…
Now, Gideon thinks he can use the tiny twins to blackmail Stan into handing over the Mystery Shack. Â He may let Mabel go if she agrees to become his queen, but Mabel won’t hear of that since we live in a democracy. Â Gideon leaves the kids in his room and sets a guinea pig to guard them. Â That doesn’t work out since it’s still a guinea pig.
There are a couple things working in the twins’ favor.  For one, Stan and Soos are setting up a mirror maze in a back room of the Mystery Shack.  For another, Soos finds Stan’s fez unattended and tries it on with a wistful, “Someday…”  And finally, the twins witness Gideon hates being tickled.  Stealing  a balloon from the used car lot out front, the twins race back to the Shack and find…Gideon has just shrunk Soos.  Stan is nowhere to be seen.  That mirror maze slows the little psycho down, and that actually leads to the twins reconciling because Mabel only ragged on Dipper’s height because he seemed to be better than her at everything else.
So, some quick thinking of an unexpected tickle to Gideon’s armpit allows the twins to get the flashlight back and return to their normal size. Â Gideon goes home, determined to get the Mystery Shack because something important is there.
And then Stan really does win that Sweepstakes, but gives it up because he thinks it’s another trick. Â The second place winner was Old Man McGucket, who wants to eat the giant check.
Those folks are right to not go back to Gravity Falls.
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