Dipper and Mabel, upon entering adulthood, immediately move to Gravity Falls with their Grunkles. Except, Dipper hears a voice in his head, which leads him to making a deal with Bill Cipher. You expect this to be angsty or sad, but no, they just have a blast going on adventures and enjoying each other's company. And somehow, romance ensues, along with Dipper accidentally becoming the new leader of the Mindscape and friends with every creature there. Bill still isn't sure how that happened.