Everything You Know Is A Lie

It's been three years since Mabel and Dipper Pines defeated Bill from taking over Gravity Falls. Stanford let the Mystery Shack business stay open for pay, even letting Stanley keep up the management.
It's been three summers since Dipper saw Bill again after he and Mabel banished him back to once he came, three summers more with his sister spent in Gravity Falls, Oregon. Three summers more he spent studying with Stanford.
He thought that now, things couldn't get any weirder in everyday life during those summers, but when a blonde haired human shows up on their front porch covered in mud, blood, and only God knows what else- claiming to be Bill Cipher himself.