Contains spoilers for this episode and earlier episodes.
Dipper wants someone to play Dungeons, Dungeons, and More Dungeons with him, but Mabel and Stan dismiss its complicated rules. Ford shares his enthusiasm and becomes the opponent he has been missing. Their game grows across the basement and into the television room, where it interferes with plans to watch the Duck-tective finale. Stan resents the intrusion, while Dipper resents being mocked for something he enjoys. During the argument, Ford's infinity-sided die falls and opens a route for Probabilitor, the game's villain, to enter the real world. The wizard captures Dipper and Ford, planning to eat their intelligent brains. Stan, Mabel and Grenda follow to rescue them.
Probabilitor makes the rescue depend on a real version of the game, where imagined spells affect the fight. Stan and Mabel discover that inventive improvisation matters as much as mathematics. They fight through his creations, and Stan rolls the unlikely number needed to beat the final monster. Probabilitor returns to his dimension, leaving the family free to watch television together. Stan apologizes for mocking Dipper and accepts his connection with Ford. Later, Ford explains that he has dismantled the portal because it is too dangerous to keep. Its reopening left an unstable interdimensional rift, which he has contained in a small vessel. He asks Dipper to keep that danger secret from everyone, including Mabel, and Dipper agrees.