Contrary

(Art credit @sketchy_bakery on Twitter.)

He's a renowned scientist, fallen from his high status and stuck teaching college physics.

Things are dull, and he finds himself craving excitement. Danger, even.

Along that path, he runs into another, one who wants the exact opposite. Peace is what this one desires, a place to rest.

They have nothing in common, and frankly, they bring out the worst in each other. It's always an argument with these two, and neither of them are humble enough to back down.

And yet, they always manage to come back to each other.

(Science Sans, from a pre-Undertale timeline, is the main character in this story. Underfell Sans, from Underfell, is the second main character. They fall in love.
Along the way, numerous other skeletons are introduced, making their way into the plot in some way or another. I haven't decided yet.)