Frisk has been followed around by Chara since they fell into the underground. After seeing Chara express a want for their body Frisk decides to give them their body back. Of course Chara can only have it under Frisk's watch.
Just as Frisk is getting settled on the surface. She starts having night terrors of Asriel and Chara. She packs her things and heads into Mt. Ebott to find him. When Frisk retrieves him and brings him to the surface they live a happy life and their romance grows stronger and stronger. With their friends and family behind them, nothing can go wrong.... Right?