For years, Papyrus knows, Sans has had nightmares. Horrible nightmares, nightmares that leave him crying in the dark, crippled in the mind, and barely able to function in life. And that's why Papyrus stays so happy, so bright, for Sans, to keep his brother going through the hard times.
But when Papyrus starts having nightmares, he knows something is wrong. And even if it means Sans is finally stepping up to fend for himself, even if he's finally once again filling the role of the protective older brother, Papyrus knows that something dark is coming...