The barrier is finally broken, and at last all of monsterkind are free to walk on the surface. Again. Sans the Skeleton is skeptical. Why bother getting excited for something that's just going to reset? This is not the first time the barrier is broken, and probably not the last. He feels the need to take a break for everyone else's enthusiasm and wanders to the ruins where he encounters a ghost of his past. The result of that meeting is something he has been hiding for a long time and that he's determined to keep hidden.