Bill Cipher x Dipper Pines, their story.

Dipper (Pinetree) and Mabel (Shootingstar) return to Gravity falls as young adults.
They find their old friends Wendy and Soos, who took over the shack when grunkle Stan past away from old-age.
Mabel and Dipper agreed to help out with the shack because after the death of grunkle the shack hasn't been in its best business.
But soon they find out that there is a new company in Gravity Falls who somehow knew how to summon the dream demon Bill Cipher.