you fall from the surface world an fall into the underworld i made my own entrance so papayrus hears you crash into the underworld and wakes up Sans he greets you into the underworld and that there is no going back to the surface he asks what it was like in the surface but you are silent because you are confused how you got here so he asks you on a tour and you don't respond because you are nervous and don't know if they are hostile so you go into the castle or home
if you want to continue the fan made story then tell me what you want to add into the story in the comments below!