The episode begins with everyone having a lazy Tuesday. Right after Danny makes a comment about how it's nice to have an uneventful day for once, Fuch appears and wrestles with a green octopus creature called the Stinky Worms to the admiration of Dipper and disgust of the others, though Mabel asks if she can keep it. Fuch takes care of the creature, and Dipper approaches Fuch, asking him if he can spend time with Ford in the basement. However due to his previous agreement with Stan to keep away from the kids, Fuch denies his request much to Dipper's disappointment, although Danny is excited, since the season finale of Duck-tective is airing on Friday.
While Mabel prepares for the airing of Duck-tective, Dipper introduces her to his favorite board game, Dungeons, Dungeons, & More Dungeons. Mabel is drawn to the game by the unicorn and "hot elf" on the box of the game but is bored by it when Dipper explains the laborious pen-and-paper process of actually playing. Soos walks by and Mabel tells Dipper to play with him instead. However, Soos doesn't like paper-and-pen games such as Dungeons, Dungeons, & More Dungeons, and instead participates in games of FCLORP (Foam and Cardboard Legitimate Outdoor Role Play) with Toby Determined, Deputy Durland and Sheriff Blubs, dressing up in cardboard and roleplaying with them. Stan walks by and makes fun of the game's nerdiness and antiquated language, with Mabel joining in, Dipper then snaps at both of them. Dipper decides to leave the room afterwards.
Dipper goes out back and tries to play with Gompers, but Gompers ends up chewing on the 38-sided die needed to play the game. Dipper wrestles it out of Gompers' mouth, but the die lands under the porch of the Mystery Shack. Dipper goes to get it, but the ground gives way and he falls into the basement, accidentally freeing the Cycloctopus. Ford walks by and begins to scold Dipper, but stops when he sees the 38-sided die Dipper has with him. Ford tells Dipper that he used to love playing Dungeons, Dungeons, & More Dungeons as well, and the two begin to play together at once.
Dipper asks Ford what he's working on down in the basement and what's behind the curtain behind them. Ford doesn't answer any of Dipper's questions, but he does show Dipper something he recovered from between dimensions - an infinity sided die that can do anything depending on what it rolls. That night, Dipper lies awake thinking of potential strategies he could use to beat Ford, keeping Mabel awake. Mabel comments on how Dipper's been spending a lot of time with Ford lately, and Dipper says it's nice to have someone that doesn't make fun of him all the time. Mabel lies awake thinking about what Dipper says.
By the next day, Dipper and Ford's Dungeons, Dungeons, & More Dungeons game has stretched across the entire basement, spilling out into the TV room and interrupting Stan, Mabel, and Grenda, who are about to watch the Duck-tective episode. Stan and Ford have a lengthy argument about who should have the room, which ends when Stan grabs Ford's dice pouch and causes all the die inside to tumble across the ground, including the infinity sided die, which zaps the box of Dungeons, Dungeons, & More Dungeons. A wizard known as Probabilitor the Annoying, an ogre, a hot elf, and a griffin appear from the box in real life. (Real life is known as dimension 46'\ to Probabilitor.) Probabilitor captures Dipper and Ford in order to eat their brains and gain their intelligence. Stan, Mabel, and Grenda decide to go on an epic wizard quest to rescue them, equipping some weapons.
Deep in the forest, Probabilitor has tied Dipper and Ford to a tree. He commands the hot elf to light the brain-cooking pot where, predictably, Dipper and Ford's brains will be cooked. Meanwhile, Stan, Mabel and Grenda make their way through the forest, Grenda defeating Probabilitor's only guard, and ultimately find Dipper and Ford right as Probabilitor is giving them an inane math problem to solve. In response, Probabilitor decides to challenge Stan, Mabel, and Grenda to a match of "Dungeons, Dungeons, & More Dungeons: Real Life Edition," which takes place on a holographic battlefield. Stan, Mabel, and Grenda control Dipper and Ford (who have been miniaturized and given fantasy outfits) and Probabilitor controls his own minions (who have been given the same treatment).
Probabilitor gains initiative and has his ogre-minions attack Dipper and Ford. Stan and Mabel are confused as to how to play, but when they learn it not only relies on math but on also on risk and imagination, Stan and Mabel realize they are in their element. The two think of cool weapons for Dipper and Ford, which they use to defeat Probabilitor's minions. However, Probabilitor reveals to the group that he is playing the controversial 1991-1992 edition and summons the Impossibeast, a monster that can only be defeated if a 38 is rolled. Stan rolls the die and gets the 38, defeating the Impossibeast and winning the game. After the game is won, the characters and the battlefield disappear. It is later revealed that Stan rigged the die with bubble gum to allow him to get the 38.
Stan apologizes to Dipper for making fun of Dungeons, Dungeons & More Dungeons and tells Dipper that he won't stop him from playing it with Ford. Everyone decides to watch the second airing of the Duck-tective episode, where it is revealed that Duck-tective has been mortally wounded by his surprise twin brother. No one is surprised at the twist ending. Ford takes Dipper into the basement again, locking the infinity sided die in a glass cabinet and telling him he can use it when needed. He also shows Dipper the thing behind the curtain - the wreckage of the Universe portal.
Ford has dismantled the Portal and reveals why he was angry at Stan for reactivating it - it has created an interdimensional rift that could lead to unknown horrors and is incredibly dangerous. Ford was able to contain it since it is a small rift, but there is a possibility of it escaping containment, so he must be careful. The episode ends with Ford asking Dipper not tell anyone about the rift, not even Mabel. Ford seals the rift away in a sliding cabinet and kill himself
As the credits roll, Soos is participating in a game of FCLORP when Durland suddenly attempts to have an insightful epiphany about their stunted development as human beings and how they use roleplaying as an escape mechanism, however Blubs reminds Durland of his place as a fortress, thus prohibiting him from speaking. Toby is carried off by the griffin, and everyone is too lazy to go rescue him.