Imagine one of your friends inventing a machine out of this world. A machine that allows you to dive into parallel worlds of earth, this means that migrating to another world similar to earth is indeed possible.
You and your friends drove and entered parallel worlds to visit as sightseeing, enjoying life as you all experience the impossible, you all had fun and laughter until.
One of you dies and disappears, when you wake up the next day, almost everyone in the friend group forgets that person.
It was like they experienced some psychological horror in one dive, wiping out their memory of that place, you know it too as your memory of that time is blurry, you can only remember words and be subjected into a room along with everyone else and locked in cages, taking a turn.
You realize this and grasp your heart, that person who disappeared was the creator of this parallel world dive, his mysterious disappearance is not bound to be simple, he could be captured by the goverment and threatened to build the machine, he could have left the country and hid abroad leaving none of his trace, and the last thing possible to happen.
He dies.
That's right, you can die inside the dive, and if you die there, then you will become a ghost staying in the same place.
Thinking about that, you cried and tried to find him in all of the coordinates of all designated parallel earths. But why jump into the conclusion of freeing him from the government or finding him across the world? The answer to that is simple, if the government truly tried, then it's not just him, it was everyone who will be dragged along the mess, plus they don't have the courage to search inside the government's facilities all over the country and the manpower to search for one man across the globe.
From there on everyone has been reliving these memories again, returning to what they have started in reverse.