Prequel - A Girl Named Jane

2013 - Twins, Dipper and Mabel Pines are back in Gravity Falls for another summer of chicanery with their friends and family. However, the town isn't as bright as they once remembered. Townsfolk are disappearing and the twins discover why after chasing up the missing Northwests.
1983 - Hawkins, Indiana and super nerds Mike, Dustin and Lucas set out to find their friend Will who vanished. Their search is somewhat disrupted by the untimely appearance of a strange girl with strange powers and a set of twins from another time.
Together this ragtag bunch of kids must not only save their towns from the horrors of the Upside Down, they must also work out why their two towns are connected...