After Mabel and Dipper's 19th birthday, something....someone is lurking thorugh the deep, darp depths of the forest watching 2 teenage pupils laugh and smile at eachother while the lurker was infuriating with jealousy. They chuckled, "You'll soon be mine, dear Pinetree." And off he went to go figure out a plan to capture (and claim) his dear ''Pinetree''.
(This takes place after Weirdmageddon was finished, Stanford and Stanley went to adventure the sea, Soos is the manager in the Mystery Shack, and Wendy moved in with her new-boyfriend, Kyle, but for Dipper and Mabel, they both stayed. And they're both glad they did.