The Last Mabelcorn
Mabel and her friends go on a magical quest to get unicorn hair in order to protect the Mystery Shack from Bill Cipher. Meanwhile, Dipper finds out that Bill and Ford used to be partners and designed the portal together. However, Bill tricked him the whole time, so Ford had to shut down the portal. Bill has been waiting for the reactivation ever since, and once he gets his hands on the interdimensional rift created by the portal it will mean the end of the world.