Sixty years ago, Frisk broke the monsters out and to the surface. The few humans there are decent to them, and the monster kingdom prospers. However, there are several barrier entryways, dotting the Earth all over the place. Asgore sends in teams to retrieve the technology to create their own barrier to prevent future tragedy.
They find out the hard way just how dangerous the barrier humans are. Sold as slaves by their souls or killed for disobedience, Sans is the only one left of the team after four months. With his own luck running out, and believing he’d be meeting his own end soon, he senses something he hadn’t sensed for sixty years.

Frisk.