You walked through the gate to Snowdin after… conveniently made Toriel kill herself(basically toriel used a huge ass fire ball to kill the player but the player dodge and the fire ball hits the ceiling which caused a massive rock to fall on Toriel and kills her). Anyways, you walked through bunch of piles of snow until you reach a bridge, which some pillars blocking the path, but the gap is too wide. Before you could go, someone approaches you… and speaks.

“Human. Don’t you know how to greet a new pal? Turn around and shake my hand.”

And when you do… BZZZ!! Shocked.

“he he he, the good ol’ hand buzzer. works every time… you don’t even have money? i wanted to go to grillby’s the old times sake. well here, have some money in store. you’re a human aren't ya? i’m sans. sans the skeleton. well, how usually goes is, i kill you, end of story. however, i’m the type of guy that doesn’t like doing things that don’t interest me, and there are things i need you to do for me. but, before i tell ya what that is, my brother should be here any minute now. well he… he’s a psychopath. it’s best for you to find somewhere to hide. and i’ve got the place. head straight.”

Well, Sans offers you a place to hide. Accept and follow, or stay and wait?