Insane? What. Not me

This story takes place after dipper and Mabel to decide to go back to gravity falls after they both turn 18. They only came back because they heard that a family member was getting sick. While the other was no where to be seen. While slowly staring to get settled into living at gravity falls something strange happens. Is dipper slowly going crazy? Or is bill actually back without anyone knowing?

These characters do not belong to me. I tjust had the idea of a story plot line that I want to try out.