They fell from the sky, only to have their landing broken by flowers carefully tended to by a crystal pony. The pony was scared away. They were scared stiff, but there was no running. The pony world they found themselves in was very real.
Sans, desperate to find some way of stopping the human's endless slaughter, forms alliances and takes drastic measures. But new variables are introduced... ones that could change everything.
WARNING: If you did not play the game Undertale (both endings), you will be VERY confused. Frisk made many mistakes for doing the genocide route in her timeline. But as if by miracle, she was sent to another world before making the worst choice ever.