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 Stanowar, 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 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. Bill leaves through the portal, telling Ford that things can 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 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 only be able to interact with people in the 'mindscape', but 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. Mabel runs away, crying. 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 a trick. He's willing to give the girls some, 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 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, Mabel storms into the glade and yells at Wendy. Mabel takes the scissors, 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 person 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. Mabel screams and punches 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 what his past is 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 was a shrine to him. Dipper takes the rift and shields it from Ford, who begins walking towards Dipper and yells at him to give him the rift. Dipper frantically asks questions to Ford about his involvement with Bill, as he picks up a memory erasing gun and points it at Ford, prepared 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. 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 of summoning Bill. When Ford read them aloud and he went to sleep, Bill appeared in his mindscape. He lied to Ford, telling him that he chose one great mind a century to inspire. Bill subsequently made a deal with Ford. Bill would help Ford with his research, but Bill would have free access to Ford's mind. At first, it seemed like Bill was helpful. Although McGucket was wary of him, the Universe Portal was completed with the guidance of Bill. But during the test run of the Portal, McGucket was accidentally transported through the Portal and found Bill's true colors. Ford confronted Bill, and Bill admitted he lied to him. 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. However, the dimensional rifts could be the next best thing for Bill to enter through. Once Bill gets 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 both will 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 says thanks, but also says she realized that morality is relative. As if on cue, Stan runs into the room and takes 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. Bill is revealed to be watching the Shack in the Nightmare Realm. 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 all the citizens of Gravity Falls as he searches for his next target.
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- Ford's middle name is Filbrick, after his father.
- Ford has a metal plate inside his head to protect himself from Bill.
- Mabel had a deep obsession with unicorns until this episode.
- Mabel's first word (according to her) was "unicorn."
- Mabel has been banned from a petting zoo for taping a traffic cone to a horse's head to make it look like a unicorn.
- Bill is older than our galaxy.
- Bill and Ford designed the portal together.
- Ford feels guilty about what happened with McGucket.
- Ford's Electron Carpet first seen in "Carpet Diem" returns.
- The interdimensional rift revealed to be created by the portal in "Dungeons, Dungeons, and More Dungeons" appears.
- Stan is seen pug trafficking, first mentioned in "The Stanchurian Candidate."
- Dipper and Mabel recall their defeats of Bill in "Dreamscaperers" and "Sock Opera."
- The door to Ford's private study from "Not What He Seems" appears.
- The Nightmare Realm, which Bill first brings up in Dipper's and Mabel's Guide to Mystery and Nonstop Fun!, gets further mention as well as a few glimpses.
- When Dipper is plugged into Project Mentum:
- Scenes from "Tourist Trapped," "Into the Bunker," and "Not What He Seems" appear in Dipper's thoughts.
- Text of Dipper's thoughts "I miss Tyrone," who was the first Dipper clone from "Double Dipper."
- Text of Dipper's thoughts "Who stole the capers?" is pertaining to a book from The Sibling Brothers series called "The Case of the Caper-Case Caper" which he was seen reading in "Carpet Diem". Another book from the same series, "The Telltale Fable of the Unstable Table," can be seen in this episode on top of Dipper while he sleeps.
- Text of Dipper's thoughts "Eeeny meeny miney you" is what Stan said when making Dipper put up the signs in "Tourist Trapped" and what Bill said before he took Dipper's body as a puppet in "Sock Opera."
- Text of Dipper's thoughts "That red bathing suit, man!" refers to Wendy's lifeguard outfit in "The Deep End."
- Text of Dipper's thoughts "I should really try out the president's key sometime." refers to the President's Key that Quentin Trembley gave to Dipper in "Irrational Treasure."
- Audio and text of Dipper singing the song Disco Girl from "Dipper vs. Manliness."
- Ford's thoughts contain Stan's line, "My brother is a dangerous know-it-all," from "Dungeons, Dungeons, and More Dungeons."
- One line of text in Ford's thoughts contained the name of his and Stan's childhood bully, Crampelter, who was seen in one of Stan's memories in the episode "Dreamscaperers" and was seen again while Stan was retelling his backstory in "A Tale of Two Stans."
- Ford's initial trust of Bill could briefly be seen in one section of the Journal 3 entry seen in "Dreamscaperers" in which Ford fondly regards Bill as a friend before being scratched out and replaced by a warning not to trust him.
- The Unicorn's entry of Journal 1 was briefly seen in "Not What He Seems."
- The rave music one of the unicorns play is the theme from Dancy Pants Revolution in "The Inconveniencing."
- Before Dipper triggers the memory erasing gun, he says "trust no one." This is one of the first things he reads when he finds Journal 3 in "Tourist Trapped."
- The cave wall Ford is seen in front of in a flashback contains the phrase "... when gravity falls and the earth becomes sky, fear the beast with just one eye." McGucket says this in "A Tale of Two Stans."
- Bill's plan was briefly alluded to in Dipper's and Mabel's Guide to Mystery and Nonstop Fun!
- A prototype of the Quantum Destabilizer (A.K.A. Experiment 618) can briefly be seen on Ford's desk in his room near the beginning of the episode before Bill enters his mind.
- The people Bill shows are, in order:
- Wendy; Old Man McGucket; Candy; Pacifica Northwest; Tyler Cutebiker; Lazy Susan; Manly Dan; Robbie; Lolph; Dundgren; Sheriff Blubs; Deputy Durland; Toby Determined; Preston Northwest; Sprott; Blendin Blandin; Shandra Jimenez; Soos; Gorney; Tambry; Bud Gleeful; Grenda; Lee; Priscilla Northwest; Mr. Valentino; Mrs. Valentino. [Once it reaches Mrs. Valentino, the list repeats itself, ending at Bud Gleeful before cutting to the end cryptogram page.]
- Mabel's' good deed list includes: save snails, plant tree, make Stan smile, donate blood, guide ducks, bezazzle Nathan, tip Susan, old lady biz, massage Waddles, sponsor clown, water tulips, take out trash, wash Soos, scratch Waddles, polish Tyler's boots, abolish electoral college, clean gutter, polish shack floor, and mowed Dan's lawn. Later, after being rejected by the unicorn a second time, Mabel also writes down: sharpen Stan's pen, fill honey jar, knit Soos a scarf, and brush a cat.
- Other good deeds were planned in production for Mabel, but they had to be cut. The deeds were: changing a car tire, giving an apology letter to Gideon for putting him in jail, paying parking meters, saving Lazy Susan's cat from a tree, writing an apology letter to Pacifica about how she "started that rumor that your butler picks your nose for you," filling Soos' cabinet with cereal, and chasing after a garbage truck for the plastic bottle she didn't recycle. Furthermore, when Mabel put the two snails on the plant, they were promptly eaten by a dog. On the list of "Bad things I did" is written that she brushed Waddles' teeth with Dipper's toothbrush.
- The "Cipher File" contains the following piece of information about pyramids:
- Pyramids also known as "Square cones", are found all around the globe and have deeply mysterious origins, engineers marvel at their seemingly impossible construction, but many don´t realize they are actually just the skeleton remnants of an ancient race of large triangular dinosaurs who had very blocky bones. The Cycloptostoneosaurus was a feared predator and roamed the plains of Northern Africa where it subsisted on a diet of warthogs and meerkats.
- Is made tender by their carefree lives and trouble-free philosophies. The reign of the pyramid continued well into the 18th century where a young George Washington once saw one on summer vacation and swore that he would found a nation with the sole purpose of putting an image of it on the back of dollar bills. Pyramids today are mainly tourist attractions and settings for conspiracy movies. Like that one were the guy has to steal the Declaration of Independence. You know the one, "Conspiracy Hank goes Overboard". If you ever get a skateboard go to Egypt and try riding downside a pyramid, I hear that's really encouraged.
- The fight between Mabel, Wendy, Grenda, Candy and the unicorns was originally going to be included but was cut due to time constraints.
- The cryptogram in the end credits is S UPYTYH DIP GAVO QETHI MCBK OHK XEXJB VRW YOUWCHIA VRSV OQ LRDIA, which when decoded with the Vigenère key SCHMENDRICK reads, A SIMPLE MAN WITH EAGER EARS MAY TRUST THE WHISPERS THAT HE HEARS.
- The key is found on a stump when Candy, Grenda, and Wendy enter the Gnome's tavern.
- The cryptogram in the episode's end page is 15-10 21-15-8-16-19-6'5 17-23-11-19 16-19 10-19-19-20-5 23 8-23-1-10, 22-19 5-3-6-19 4-9 13-10-9-1 1-16-15-21-16 5-15-20-19 25-9-3'6-19 9-10 which, when decoded using the Combined Cipher, reads: IN CIPHER'S GAME HE NEEDS A PAWN, BE SURE TO KNOW WHICH SIDE YOU'RE ON.
- When Dipper is looking through The Cipher File under "Mind Scape" there is a cryptogram that reads " & ." Once decoded using The Author's symbol substitution cipher, it reads "BLACK & WHITE."
- The many codes and cryptograms found on Stanford's chalkboard are as following (next to the cryptograms are the decoded text using The Author's symbol substitution cipher. Separated by a semicolon means they are written above each other):
- On the stone wall in the cave Ford found there is a cryptogram that reads: "." Once decoded using The Author's symbol substitution cipher, it reads "IT STARTED WITH BAD DREAMS WHICH BECAME NIGHTMARES. I WAS FOOLISH, I WANTED ANSWERS, I PAINTED THE SYMBOLS, I SAID THE WORDS: WHEN GRAVITY FALLS AND THE EARTH BECOMES SKY FEAR THE BEAST WITH JUST ONE EYE."
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