In this fanfiction, the plot might center around Sans and Papyrus's reaction to seeing Gaster again. They could be shocked, scared, or even a bit angry if Gaster had left them before without a proper explanation. The story could explore their relationship, how it was in the past, and how it changes as they interact with Gaster now. It might also involve other characters from the Undertale universe reacting to Gaster's return and his interactions with Sans and Papyrus.
Perhaps the plot involves Gaster coming back with a warning. He could be warning Sans and Papyrus about an impending danger to their world. They then have to figure out what to do with this new information and how to protect their friends and home. The story might show their journey of discovery as they try to understand Gaster's motives and the nature of the threat.
Well, it could be about Gaster suddenly reappearing in the lives of Sans and Papyrus. Maybe he has some important knowledge or a mission for them. For example, he might need their help to fix something in the Underground that's going wrong.
Obviously, Sans and Papyrus are the two main characters who we already know well from the original Undertale. They have distinct personalities that will play a big role in how they interact with Gaster. Gaster, on the other hand, is the enigma. His presence will likely drive the story as Sans and Papyrus try to figure out who he is now and what he wants, so he's also a main character in this fanfiction.
Another plot twist could be that Papyrus realizes the fight is all an illusion. In the fanfiction, he might see through some sort of magic or mind - control that was making Sans fight. So, he then has to figure out how to break the illusion and save Sans from this false battle.
One common theme is mystery. Since both characters have elements of mystery in their original lore, fanfictions often explore that further. Another theme is power dynamics. Given their different abilities, how they interact in terms of power is often explored. And friendship or a deeper connection is also a common theme as writers like to explore the relationship between these two unique characters.
One common theme is mystery. Since both Gaster and Sans have mysterious backstories in the original game, fanfictions often explore and expand on these mysteries, especially in relation to their relationship.
A plotline could be that they discover a hidden part of the Underground. This new area could have different rules and strange creatures. They have to adapt quickly to survive and also figure out how this new place relates to the rest of the Underground.
A great idea could be a story about their daily adventures in the Underground. For example, they could be on a quest to find a rare ingredient for a new recipe Papyrus wants to try. This would focus on their brotherly bond and Papyrus's enthusiasm for cooking, while Sans tags along with his laid - back attitude.
One great idea for a fanfiction could be a story where Underswap Sans and Underswap Papyrus go on an adventure in the Underground. Maybe they encounter a strange new monster that no one has ever seen before. They have to use their unique abilities to figure out how to deal with this new threat.
Papyrus might be initially shocked. He's used to Sans being more laid - back, so seeing him in a fight could be quite a surprise. His eyes would widen and he might gasp.
One possible reason could be for some sort of experiment. Gaster is often depicted as a mysterious scientist in the Undertale lore, and Sans has unique abilities. Maybe Gaster wants to study Sans' power over time and space. Another reason could be related to secrets. Sans might know something about Gaster's past or the true nature of the Underground that Gaster wants to keep hidden, so he kidnaps Sans to prevent him from revealing it.
A sense of otherworldliness is common too. Since Gaster Sans is from a rather strange and unique universe, the fanfictions often play with the idea of the reader being in a very different, almost alien world. This can lead to themes of adjustment, as the reader tries to fit into this new world while being in a relationship with Gaster Sans.