Contains spoilers for this episode and earlier episodes.
Mabel discovers that she has grown one millimeter taller than Dipper and spends the day celebrating her tiny advantage. Dipper, accustomed to beating her at games, becomes increasingly annoyed by her jokes. He consults the journal and finds crystals that alter the size of anything touched by their light. Mounted on a flashlight, they let him make himself taller, but Mabel discovers the trick and fights him for the device. Gideon takes advantage of their argument, steals the flashlight and shrinks both twins. He keeps them in a jar while planning to use the same power against Stan and seize the Mystery Shack.
Dipper and Mabel escape Gideon's room and follow him back to the Shack. Their first attempt to recover the flashlight fails because they are still arguing about height. Soos also becomes trapped at their scale, and Dipper finally asks Mabel why the teasing mattered so much to her. She explains that being taller felt like winning something after losing so many competitions to him. They apologize and work together as Gideon searches for Stan in a mirror maze. The tiny group climbs onto Gideon and tickles him until he drops the flashlight, making his threats look ridiculous to Stan. The twins restore themselves, with Dipper allowing Mabel to keep her extra millimeter, then destroy the dangerous crystals.