Home and Home Again

After escaping the Underground and getting clipped by a truck, an amnesiac Sans ends up in the home of a vet, a doctor, and their daughter. He's certainly nothing they've ever seen before, but they can't just let him suffer either, so he becomes the unlikely son. So when Frisk falls into the Underground, Sans must return home to the place he was born and raised. Can he and Frisk free monsterkind from the mountain, or will they make a home Underground with the family Sans lost long ago?