The episode begins with Dipper and Mabel, excited for their thirteenth birthday. Mabel goes to tell Soos and Grunkle Stan, only to find out that they have more things. She goes to Wendy, along with their friends, disappointed to go back to high school as the summer ends, even Candy and Grenda can't come due to their problems (Candy go to the girl scout camp as Grenda is in date with Marius), much to her disappointment.
Dipper goes to get Ford for their birthday, only to find out that he is trying to go to find UFO in the cliff, where it crashed, there they find some robots trying to capture Ford (they have hunted him down for all those years). They have defeated him, with Ford, impressed he wants to make Dipper as his student. Dipper decides to go and tell Mabel about.
After returning to the shack, he finds out that Mabel overheared their conversation through the RC. Mabel is despodent that they are going to be 13, summer ends and she has to left everything behind forever. Heartbroken, Mabel wants that the summer could last forever. Dipper sympathetically apologizes to his sister, saying that he will ever come to them always. Dipper attempts to comfort her, saying when they have all fun things, summer ends. Mabel grabs her backpack, runs away to the woods, crying, with Dipper asks her to come back.
Mabel above the tree, sitting, she realizes she grabbed the wrong backpack. She laments until "Blendin Blandin" unexpectedly arrives and asks her the globe that could change reality, and summer could never end. However, she is unaware that Blendin is actually Bill Cipher! He broke the globe, unleashing the monsters from Weirdmageddon.
Dipper comes back, disappointed that he did the wrong choice by choosing to be Ford's student and for breaking his sister's heart. However, to their horror, they see monsters come out from the unknown portal in the sky. Ford declares that the Weirdmageddon begins as the episode ends.
- It is revealed in this episode that Dipper and Mabel's birthdays are on August 31.
- At the end of this episode, Bill, possessing the body of Blendin Blandin, shatters the rift, beginning Weirdmageddon.
- Doge appears on the calendar when Mabel is talking about Soos, Grunkle Stan, her, and Dipper's big day.
- Right before Bill reveals himself and comes out of Blendin's body, he falls on the rift, but later, it is bare. Also, Bill took off his glasses, but when he is rising to the sky, Blendin is wearing his glasses again.
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