Who?
After the explosion of the gateway, the author of the Journals has returned from 20 years of running in alternate dimensions. It's also revealed to us that the author's name is Stanford Pines, and is the brother of Stanley Pines, who took his brother's name in order to use his research lab as the mystery shack. Now, back from the horrors which he faced, Stanford is back in business, and he intends to shut down Bill Cypher forever.
Story
Bill Cypher
After the reunion with each other, the author explains to us that 20 years ago, he asked his brother to hide Journal #1. Unfortunately, a fight broke out between them, and by accident, Stanley pushed Stanford into the open gateway, sealing him away. For the past 20 years, Stanley has been trying to find all three of his brother's journals to find out how to operate the machine. Now, in the present day, Stanley has managed to reopen the portal and bring back Stanford, however, in doing so, he has created something called an 'Intergalactic Rift'. This vortex encased inside of a reinforced glass orb is a bit like a portable gateway, only if it breaks, the dimensions will be connected forever. As much as he fears it, Stanford knows that Bill will do practically anything to get a hold of this trinket, and in order to do so, he would have to possess people that he loves. To avoid that fate, Stanford decided to build a protective aura around the mystery shack, making it impenetrable to Bill's powers. The next thing that the author had to take care of was the rift. In order to make sure it wouldn't collapse, Stanford and Dipper went on an expedition to an alien crash site. There, they found an adhesive strong enough to mend the Earth together. Now all there is to do is seal the intergalactic rift back together.
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But something went wrong.
What Happened?
At one point in the story, while it's not explicitly told how, Bill Cypher manages to convince Mable into giving him the intergalactic rift. In doing so, Bill Cypher has connected his galaxy with the regular one, and now all the terrifying creatures of alternate dimensions can start to pour into the world. Everyone is confused, Mable is missing, and the world is at stake.
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Who's going to save us now?
Overview
The Intergalactic Rift
In brief, the original author of the journals has returned, and with him came the legacy of saving the world for Bill Cypher. As a first step, the author dismantled the gateway his brother used to get him back from alternate dimensions. Next, he found out that the re-activation of the portal formed a small rift, which he decided to conceal. However, after finding something which would have been able to seal it shut, Bill Cypher managed to get ahold of the delicate item. Now, the city of Gravity Falls is left at mercy of Bill and his friends from other dimensions, and it looks like there's going to have to be a someone to take care of it.