Each world has its own key, and when you get that key, you can open the door and travel through the world.
To get the key, you must first——
There is that door.
The young man with a bunch of shiny keys hanging around his neck, looking at the unconscious visitor from another world in front of him, finally couldn't help but look up to the sky and roar——:
"Can this world that is broken into a sieve get better?!"
