The monsters have been released from the Underground, and are planning on making peace with the humans. Frisk promises to be the ambassador of the Underground residents, and as the years go by, the humans and monsters slowly become friends again. Frisk continues to be the ambassador for the monsters, learning more about her friends and herself as she gets older. Sans has had feelings for her for awhile, and he's been afraid to confess them to her. But he'll have to soon, because while Frisk grows older, gets married, has children, and lives her life, he stays young.
I'm usually not one for writing fanfictions, but after playing the game, I couldn't help but ship these two characters. And a friend of mine asked/dared me to do it.
Note: Cover is not mine, and belongs to lacampanellanuraver on DeviantArt