The Last Mablecorn

The episode starts at night, showing Dipper and Mabel sleeping in the attic. Ford is also sleeping in his old room, and begins to dream. In his dream, Ford is standing in a field of wheat, with a swing set, the ruined Stan 'O' War, and the ruined Universe portal in the background. Ford stares in shock as the wheat is flattened into the pattern of Bill Cipher.

Bill appears to Grunkle Ford, calling him his "old pal." Bill says that he's been going around making deals and preparing for the "big day." Bill grabs a copy of the interdimensional rift, warning Ford that he can't keep it safe forever. Bill throws the rift down to the ground, opening a red triangular portal in the sky and burning the wheat field. Ford yells at Bill to leave his mind, telling him he has no dominion in the real world. But Bill tells Ford that "not right now" for dominion in the real world and things can change and leaves through the portal to the Mindscape saying, "things change". A series of cryptic images flash as Ford wakes up, telling himself he has to warn his family.

In the morning, Dipper and Mabel try and find a board game to play, as Stan continues his pug trafficking scheme. Ford calls a family meeting, and Dipper and Mabel attend while Stan ignores. Ford asks the twins if they know about Bill Cipher, to which the twins explain their last two encounters with him. Ford, slightly shocked, explains that Bill is growing in power. Luckily, he has a way to shield the Mystery Shack from Bill. To do so, he needs several magic ingredients, including unicorn hair. Ford, looking through *Journal 1*, shows how unicorns reside in an enchanted glade, and that their hair can only be taken by the pure of heart. Mabel jumps at the opportunity, saying she loves unicorns, and she's probably the most pure of heart in the room. After no objections, Ford agrees to send Mabel on the quest and gives her a crossbow and Journal 1 to help her. Mabel calls Candy, Grenda, and Wendy to help her out. After Mabel leaves, Dipper asks Ford what the plan is to stop Bill. Ford brings Dipper to the second level of the basement, where Ford has made a private study. No one, not even Stan, knows it exists. Ford tells Dipper that if Mabel can't get the unicorn hair, they'll have to block their minds from Bill instead. He shows Dipper a network of terminals, with a helmet connected to it.

Meanwhile, the girls continue their quest. Mabel, Candy, and Grenda are all excited to see a Unicorn. Wendy claims she doesn't believe in unicorns, and only agreed to come along to keep the kids from getting into any trouble. When the girls step into a magical area in the forest, Mabel consults Journal 1 and finds that to summon a unicorn, one must utter a deepest chant once used by ancient druids. After Grenda successfully does so, the glade rises up out of the ground, and the girls enter.

Inside, sitting on a rock, is the unicorn Celestabellebethabelle, apparently the last of her kind. Mabel steps forward and explains that she needs a piece of her hair to protect her family. Celestabellebethabelle examines Mabel's heart and comes to the conclusion that she is not pure. The unicorn tells Mabel to come back when she is pure of heart. Distraught, Mabel realizes that Celestabellebethabelle was right and that she has been "slacking off on the whole goodness department." Mabel decides to do a ton of good deeds to make up for it. Meanwhile, Ford flicks the switch on the terminals and puts the helmet on Dipper.

Back at the Mystery Shack, Dipper asks Ford what Bill really is. Ford answers that no one knows for sure, but they do know that he is incredibly ancient, "older than our galaxy and far more twisted." Bill comes from an alternate dimension known as the Nightmare Realm, but he doesn't have a physical form on Earth. He is limited to interacting with people in the 'mindscape', but he could use the dimensional rifts to open a portal straight to Earth. This is why Ford disabled the Universe Portal. Ford tells Dipper the machine he's hooked up to is designed to scramble his thoughts to outside entities so that they can't interact with his mindscape. The terminals show Dipper's thoughts as this process occurs. When Dipper asks Ford what his history with Bill is, Ford dodges the question and tells Dipper to focus on the machine for now. A montage occurs as Mabel performs many good deeds throughout Gravity Falls, with sometimes unintentional results.

Eventually, Mabel succeeds in performing 1,000 good deeds, believing this will make her pure of heart in Celestabellebethabelle's eyes. However, the unicorn still says she is not pure of heart. She explains that doing good deeds to make yourself look better isn't pure at all and that Mabel is literally a bad person. Devastated, Mabel runs away, sobbing. Outside the glade, Mabel is curled up into a ball. After the girls give Mabel an unsuccessful pep talk, Mabel tries to brainstorm good ideas to make her pure of heart. Wendy leads Candy and Grenda away from Mabel and tells the girls that since getting the unicorn hair the nice way failed, they should now try it "the Wendy way," without Mabel's knowledge. Wendy then comes up with a plan to get Celestabellebethabelle's hair and whispers it to Candy and Grenda.

Wendy, Candy, and Grenda break into a secret gnome tavern called Gnasty's, asking if anyone knows how to knock out a unicorn. A decrepit gnome in the back of the room tells them a bag of fairy dust will do the trick. He's willing to give some fairy dust to the girls, but he wants a bag of butterflies in return. The deal is done in secret, since butterfly trafficking is illegal. The girls get the fairy dust from the gnome and give him the butterflies. But this turns out to be a sting operation as the gnome cops come out of hiding and arrest him, with Grenda paying one of the gnomes a bribe with one of the bags of fairy dust.

At night, Mabel is still outside the glade. Wendy, Candy, and Grenda sneak up behind Mabel, throw fairy dust at Celestabellebethabelle, and knock her out. As Wendy gets a pair of scissors and prepares to cut a piece of unicorn hair, an angry Mabel storms into the glade and yells at Wendy. Mabel takes the scissors away, telling them what they're doing is wrong. The unicorn then wakes up, sees Mabel holding the scissors, and goes into a rage. She tells Mabel she will never be pure of heart as Mabel tries to explain herself.

Suddenly, two male unicorns appear from behind the forest and admonish "C-Beth" for pulling the pure-of-heart scam again. They explain to the girls that unicorns can't really see into your heart, and that C-Beth was lying. She tells every traveler they're not pure of heart to get humans to leave her alone. Mabel becomes furious, saying that C-Beth is even worse of a being than she is. C-Beth admits that what the other unicorns are saying is true, but gloats in the girls' faces, telling them they can't do anything about it. Enraged, Mabel screams and strikes C-Beth right in the nose. As the girls call for a fight, C-Beth is more than willing to oblige. The two parties yell and run at each other, getting ready for an epic fight sequence. As Dipper is hooked up to the machine, Ford is asleep at his desk. The machine shows that Dipper is thinking of using it on Ford to reveal his past with Bill. After a brief internal struggle, Dipper puts the helmet on Ford. Bill himself appears on the screen. Several images flash on the screen, including one showing a young Ford in his mindscape making a deal with Bill, calling him "a friend." Ford wakes from his sleep and stands up, staring at Dipper with shining glasses. He angrily tells Dipper he shouldn't have done that.

Ford takes the helmet off and throws it across the room, causing shades to fall off the walls. Posters and sculptures of Bill are revealed all over the walls, as if the room were a shrine to him. Dipper takes the rift and shields it from Ford, who begins walking towards Dipper, yelling at him to give him the rift. Dipper frantically asks Ford questions about his involvement with Bill, as he picks up a memory erasing gun and points it at Ford, preparing to wipe Bill out of Ford's mind. As Ford walks closer and closer, Dipper fires the Memory Eraser. The shot hits Ford's glasses, bouncing across the room, and hits the terminals, destroying them. Ford picks up Dipper, telling him to calm down. His glasses stop shining, revealing he has normal pupils. Dipper apologizes for almost erasing Ford's mind, which Ford accepts pointing out that due to the metal plate in his head it wouldn't have worked anyway, but Dipper would've done great if it was Bill. He reveals his backstory with Bill to Dipper, saying Bill tricked him when he was younger. Long ago, he had hit a roadblock in his investigation of Gravity Falls. He found ancient writing in a cave, incantations to summon Bill. When Ford read them aloud and went to sleep, Bill appeared in his mindscape. He lied to Ford, telling him that Bill chose to inspire one great mind every century. Bill subsequently made a deal with Ford. Bill would help Ford with his research, but in exchange, Bill would have free access to Ford's mind. At first, it seemed Bill was helpful. Although McGucket was wary of him, the Universe Portal was completed with Bill's guidance. But during the test run of the Portal, McGucket was accidentally transported through the Portal and discovered Bill's true colors. Ford confronted Bill, and Bill admitted that he had lied. He said the Portal would act as a gateway to the Nightmare Realm so that Bill and his fellow demons could show Earth "how to party." Panicked, Ford shut down the portal for good, even though Bill said that wouldn't change anything.

Ford hid the instructions from his journals so no one could finish what Bill started then dismantled the portal when he came back. However, the dimensional rift could be the next best way for Bill to enter our dimension. If Bill got his hands on the rift, it would mean the end of Gravity Falls and the world. Back on the main floor of the Shack, Ford tells Dipper that there will be no more secrets between them. They will both do anything to stop Bill. Dipper realizes they have no way to Bill-proof themselves, as he broke the machine. Suddenly, Mabel and the girls appear, covered in various unicorn fluids. Mabel managed to get some unicorn hair and a treasure chest full of unicorn goodies. Ford is ecstatic, telling Mabel she is a good person. Mabel tears up and thanks him, but says that she's also realized morality is relative. As if on cue, Stan runs into the room and steals the money from the treasure chest.

As the credits roll, Ford and Dipper finish the Bill-proof barrier. Anyone inside the Mystery Shack will be immune to Bill's mind tricks. Unfortunately, Bill is revealed to be watching the Shack in the Mindscape, and he comes to the conclusion that if his next pawn won't be inside the Shack, it'll have to be outside it. His eye flashes between numerous residents of Gravity Falls as he searches for his next target to possess and trick the Pines family when the time is right and when they least expect it.