The beginning

Before the events of Underverse begin, seven people disappear from their Multiverse. Dream, Error, Fresh, Stretch, Outer, Core!Frisk and Cross!Chara are trapped in a new world that they can't leave until they watch everything they're given. They get one chance and have to choose carefully what they change, for better or worse.

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Basically the characters watch Underverse, then XTale, then Underverse again, you get the idea