After Frisk saved the Underground, living with Toriel and Asgore was the perfect happy ending for her, despite the fact that Monsters continued to seclude themselves in the underground because of awkward relations with Humankind. But now that Frisk has gotten just a little bit older, she's found a new hobby to focus her DETERMINATION on... Sans. Except his past complicates things.

Post true pacifist ending (SO SPOILER ALERT). There are still some unanswered questions after the monsters are freed. How do humans interact with them? Why does sans have a lab? Who is W.D. Gaster? And why did Frisk go to the underground in the first place? The story continues where the games leaves off, watch all the characters you love adapt to their new lifestyle.

They were just supposed to be the parts Gaster needed to fix himself, to rebuild himself anew after the trauma the war wrought on his mind and his body, but things change... After the Barrier fell, Sans and Papyrus's souls begin to break, and Gaster comes forth to give them what remained of his own soul, with effects none anticipated.