You wake up on a bed of golden flowers and find a friend to help you through the Ruins. You narrowly avoid death by killer goat mom and have your mind set on leaving the Underground.
But then you bump into someone. He's a bit of a jerk, he swears a lot, and he looks positively terrifying. But Sans didn't kill you on sight. He also stopped his brother before he destroyed you and convinced Papyrus to keep you as their slave rather than as a dead body. But Paps makes Sans agree that you are his responsibility and it is Sans' job to look after you.
Things may seem tough, but at least you have two friends. Flowey is one. Sans could be called the other but you're constantly debating this.
Soon however, you realise something. You don't want Sans to be your friend at all. Your feelings for him are stronger than that.
(Warning: contains strong language. I mean like mega swears words. We're talking the F word, the S word... even the C word. Yep. So... read with caution if you're 4 I guess.)