Mabel decides to plan her and Dipper’s 13th birthday party. Meanwhile, Ford reveals to Dipper the most surprising secret yet – a giant UFO which, according to Ford, crashed millions of years ago and is buried under the town. It was by using parts of this alien technology that Ford was able to construct the dimensional portal. Bill Cipher tricks Mabel into handing him the interdimensional rift, which Bill then uses to enter the physical world and open a portal from his own dimension. The end of the world begins.