Dipper and Mabel started to have some problem's, they seem to have fallen ill with a mysterious sickness at the same time Stan is acting weird around them and things get worse when Dipper gets a visit from Bill in one of his dreams where he offers a portal to him that may help him understand more about this sickness a red one and a blue one.
Little to his knowledge celestia is the truth and pain of it all.
ps. sad for some sad moments.
pss. this takes place at the end of season 3.