The episode begins with Mabel waking Dipper up in the attic of the Mystery Shack. Having a pair of googly-eyes on her chin, Mabel talks as "Mr. Upsidedownington," leaning over Dipper, who tells Mabel that she hasn't done that since third grade. Mabel reminds Dipper that their 13th birthday (which is August 31st, the last day of summer) is only one week away, which Dipper had hardly realized. Suddenly, Soos and Grunkle Stan enter the room, with both of them talking about events happening in the near future. Soos remarks on how the future is coming for them all and everyone echoes him.
Dipper and Mabel head downstairs to begin planning, with Mabel believing that the big birthday party will be thrown at the Mystery Shack. However, Stan says they are not allowed to have another party at the Shack after the events of "Scary-oke." Soos recommends the Gravity Falls High School gym as a good place to have the party. The twins are about to head there together, but Ford calls Dipper over, saying his face is on fire.
Dipper runs to help Ford, but he has everything under control. He then shows Dipper something extremely urgent - the fact that the interdimensional rift is cracking. He then explains that if Bill Cipher manages to get the rift when it's completely cracked, he would unleash a catastrophic event that will unravel reality. Ford had dubbed this event called Weirdmageddon. Dipper is hesitant, wondering what he should do with Mabel and his birthday party. Mabel walks in, telling Dipper that he can go on his quest with Ford, and saying that the two of them can stay in touch using walkie-talkies. Ford runs off, telling Dipper to hurry. Dipper psyches himself up for his first mission with Ford and promptly walks into the doorframe before actually heading out the door. The two of them go on a quest to patch the rift, and Ford will explain the exact nature of his plan along the way.
Mabel and Soos head to Gravity Falls High School, where Lee and Nate are rearranging the letters on the school's sign. The two go into the gym, finding lines of despondent high-schoolers. Mabel finds Wendy and asks her what's happening. Wendy explains that it's registration day and that no one is happy about it. Mabel is confused, thinking high school is fun and song-filled, just like on TV. Wendy says that TV's portrayal is a lie and that if Mabel can avoid growing up, she should. Everyone in the gym promptly laughs at Wendy as the man in charge of registration mispronounces her name, leaving Wendy embarrassed.
Mabel exits the gym, becoming depressed. She tries to radio Dipper via walkie-talkie, saying that they can have the party at the gym but the future of high school is becoming bleak. Dipper is unable to respond, as there's too much interference on his end. Soos sits next to Mabel, suggesting that she delivers invitations to her friends. As the two drive away, Mabel sees the high school sign has been changed to read "NO ESCAPE."
Ford and Dipper reach their destination - a large hill overlooking the floating cliffs of Gravity Falls. Ford remarks on how the cliffs look seemingly unnatural and oddly shaped, and Dipper realizes what Ford means when he holds a UFO keychain in front of him. Ford reveals to Dipper that the valley of Gravity Falls was created millions of years ago, when a spacecraft of some kind crash-landed into the area, creating the shape in the cliffs. The spacecraft is now buried under the town. Ford has had a lot of experience with the craft, having raided its parts to build the universe portal. He assures Dipper that all of the aliens inside have "probably" been dead for millions of years. Ford opens a hatch on the hill using a magnet gun, and the two head down it, into the depths of the spacecraft.
Inside the spacecraft, Ford and Dipper walk around, attempting to search through its halls for the adhesive. The two pass through the craft, which is dominated by huge open spaces that contain mysterious message written in code, ruined machines, and alien skeletons. Ford tells Dipper that the spacecraft contains an alien adhesive powerful enough to seal the crack in the rift, and their job is to look for it. Ford uses his magnet gun to slide down several metal columns in order to head further into the craft, but Dipper is hesitant to follow him. He uses the magnet gun, but it ends up attaching him to the ceiling instead of pulling him down.
Meanwhile, Mabel goes to see Grenda and Candy in the hopes of giving them invitations to the birthday party. Unfortunately, Grenda will be visiting Marius in Austria, and Candy will be at music camp. Mabel is sad that neither of her best friends will be able to come to her party or wish her goodbye when she leaves for home. They share a hug, and Mabel leaves. She tries to radio Dipper, but he doesn't answer.
As they search, Ford offers to make Dipper his apprentice in learning about the secrets of Gravity Falls. Dipper is unsure since he would have to part with Mabel when she returned to California. As he speaks, he uses the magnet gun on one of the alien plates, revealing the adhesive. They are about to leave with the adhesive when suddenly, two prison droids appear. Ford tells Dipper that they focus on adrenaline, and if you are not scared, they will not hurt you. However, Dipper is unable to keep his fear in check, and realizing this Ford strikes first, destroying one of the droids with the magnet gun and the other droid attacks Dipper. Ford pushes him out of the way, but he is injured and captured by the droid. Dipper chases the droid through the saucer. The droid puts Ford into a different, ship-like droid, that will take Ford far away. As the little ship prepares to take off through a hatch in the saucer, Dipper attaches himself to the ship with the magnet gun. The ship takes off, streaking over the town and destroying the water tower as Dipper desperately tries to disable the ship. Ford hits his head in his cell-like bubble and is knocked out. Dipper uses the pulse option on the gun to disable the ship, which sends it crashing down in the woods.
After recovering, Dipper opens the pod and frantically tries to revive Ford. The malfunctioning prison droid that had captured Ford returns to take Ford and Dipper away. Dipper stands up to the droid, yelling that he isn't scared and that nothing will take away his uncle. The droid does not detect any adrenaline and disables itself. Ford finally recovers and praises Dipper.
Back at the Shack, Mabel is sad that summer is almost over and that high school is awful. Stan comes in to cheer her up, reminding her that growing old doesn't mean you have to grow up and that it's okay to have fun even as an adult. Mabel says that she doesn't want to say goodbye to Gravity Falls, but Stan reminds her that she still has her brother, no matter what. Mabel is cheered up by this. However, her walkie-talkie starts to crackle, broadcasting a conversation between Dipper and Ford.
Ford again extends the opportunity to stay in Gravity Falls to Dipper. Dipper decides to take the offer, and they return to the Shack. Dipper bursts into the attic, eager to share the news with Mabel, but Mabel already knows, having heard it on the walkie-talkie. Mabel is despondent, explaining that she had a horrible day and that when summer is over, the only thing she won't have to leave behind is her brother. However, if Dipper stays, she would have to leave absolutely everything behind. Dipper reminds her that things change, and summers end. She grabs a bag and runs away crying into the woods.
After sitting down against a tree, Mabel looks through the bag, only to find that she had grabbed Dipper's bag by mistake. She wishes for summer to last forever. Blendin Blandin appears, offering to help her make a "time bubble" that will make time stop, letting summer last as long as Mabel wants it to. All he needs is a "gizmo" from her uncle - describing the rift. She finds it in Dipper's bag, not knowing of its importance.
In the basement, Dipper wonders if he's making the right decision by staying. Ford reminds him that they need to save the world and asks for the rift. He reaches into his bag, only to pull out a birthday party invitation instead. He realizes that he has Mabel's bag - but it's too late.
Mabel hands the rift over to Blendin, who takes it and smashes it on the ground, stomping it with his boot. The liquid of the rift surges across the ground as Blendin begins to cackle, taking off his goggles and revealing his eyes to be the same as Bill's. Mabel realizes that she has made a horrible mistake, but before she can react, Bill, in Blendin's body, snaps his fingers, knocking Mabel unconscious and causing an unearthly wind to howl. Bill leaves Blendin's body and enters the physical world, screaming about how the gateway between the Nightmare Realm and the real world has been opened after being prophesied for a billion years. Bill rises to the sky as a giant cross appears thanks to the rift's influence - a gateway from the Nightmare Realm to Gravity Falls. Dipper and Ford run outside to investigate the chaos, seeing the gateway and a humongous Bill. Ford realizes that they've failed and that Weirdmageddon has begun.
The credits show one of Mabel's invitations on the ground, ruffling in the breeze. There is screaming and other sounds of distress fill the air. The paper blows away before the cryptogram appears.
- Outline by:
- Mark Rizzo
- Alex Hirsch
- Directed by:
- Additional Written Material by:
- With the Voice Talents of
- Additional Voices
- Casting by:
- Dipper and Mabel were born on August 31, 1999.
- When they were younger, Mabel used to glue googly eyes to her chin and talk as Mr. Upside-downington, but she hasn't done that since third grade.
- Instead of shaving, Ford sets his face on fire since he finds it to be faster.
- Ford has 12 Ph.Ds.
- Ford raided the UFO spacecraft for many years and even used some of the parts to create the universe portal.
- Candy's parents make her go to music camp each summer.
- Grenda finds Marius von Fundshauser clingy.
- Mabel's scrapbook of summer memories, which is first seen in "Dipper vs. Manliness" and later shown in "Society of the Blind Eye," makes an appearance.
- Grenda's pet iguana, which first appeared in "Double Dipper," makes an appearance.
- On Mabel's miniature Mystery Shack there is a banner that reads "Sev'ral Timez."
- Stan complains that he is still finding pieces of the undead under the cushions after "that zombie incident."
- Tyler Cutebiker is shown as Gravity Falls' mayor after winning the election in "The Stanchurian Candidate."
- The fishing hat Stan made for Mabel in "The Legend of the Gobblewonker," can be seen on Mabel's bedpost.
- The green bottle Mermando sent Mabel with his note (from "Society of the Blind Eye") can be seen behind the left post of Mabel's bed.
- Blendin still has the hair the twins gave him in "Blendin's Game."
- During the cold open, next to the calendar in the twins' room is a picture of Mabel and Waddles from "Land Before Swine."
- The UFO key chain seen in "The Hide-Behind," “Scary-oke” and "Dungeons, Dungeons, and More Dungeons" makes an appearance.
- Grenda is still in a relationship with Marius from the events of "Northwest Mansion Mystery."
- Bill Cipher possesses Blendin Blandin, who was one of Bill's "options" to possess, as seen in "The Last Mabelcorn."
- Near the end of the episode, Mabel goes to what she calls "Sweater Town" just like she did in "The Hand That Rocks the Mabel."
- Dipper says, "I was wrong about Stan's portal," referring to his suspicions in "Not What He Seems" when he thought that Stan was using the interdimensional portal as a doomsday device.
- Dipper also makes a reference to "Sock Opera," where he said he was tricked by Bill Cipher.
- Several chewed pens are in Dipper's backpack. Dipper's habit of chewing them was first shown in "Society of the Blind Eye."
- The end credit cryptogram, "DID YOU MISS ME?" is one of Bill's lines from "Sock Opera."
- It is revealed that the entire valley of Gravity Falls was formed when an extraterrestrial object crash-landed there millions of years ago.
- The number 816 is written on Ford's Weirdmageddon board.
- A scene where Mabel waves goodbye to Stan after their talk was cut from the episode's final production.
- Mabel thinks high school is a place where students and sing about following their dreams "like some sort of musical", as she learned watching TV until Wendy tells her otherwise. This is a reference to the Disney Channel Original Movie High School Musical and the way it portrays teenagers, which is replicated in several other Disney shows and films broadcast on television.
- When the townspeople are looking up at the rift in the sky, both of Lazy Susan's eyelids are open suggesting that either this is an animation error or Lazy Susan can open both her eyelids when she is extremely surprised or in awe.
- The symbols inside the UFO (that Dipper took a selfie with) are written in the Extraterrestrial symbol substitution cipher.
- In black writing: "," "," "," and "" decodes to: "PROBABILITY DRIVE ENGINE," "PROBE ATORIUM," "BLERG BLOTH AND BEYOND," and "USED THATENS HALF PRICE."
- In brown writing says "" decodes to: "SPECIMEN HAS ESCAPED IS CHANGING FORMS."
- In the notebook in Dipper's bag, "mycpe gt" is written in the author's symbol substitution cipher.
- Along one of the pipes in the UFO, a cipher written in the Extraterrestrial symbol substitution cipher reads, "" decodes to "BETTY AND BARNEY WERE HERE."
- The cryptogram in the episode's credits reads, "ETX CPI ASTD GI?" Once decoded using the Vigenère cipher, it reads "DID YOU MISS ME?"
- The key "BLUEBOOK" can be seen on the fences.
- The cryptogram in the episode's end page reads, "4-16-19 8-6-9-8-16-19-21-25 5-19-19-11-19-20 18-23-6 23-1-23-25 22-3-4 18-15-10-23-12-12-25 1-19'2-19 6-19-23-21-16-19-20 4-16-19 20-23-25 17-15-2-19 3-8 4-16-19 8-23-5-4. 19-11-22-6-23-21-19 4-16-19 5-4-6-23-10-17-19. 19-2-19-6-25-4-16-15-10-17 25-9-3 21-23-6-19 23-22-9-3-4 1-15-12-12 21-16-23-10-17-19." Once decoded using the combined cipher it reads, "THE PROPHECY SEEMED FAR AWAY BUT FINALLY WE'VE REACHED THE DAY. GIVE UP THE PAST. EMBRACE THE STRANGE. EVERYTHING YOU CARE ABOUT WILL CHANGE."
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